Novel activation and transfer cascade for ubiquitin

ABSTRACT

A novel activating enzyme for ubiquitin, Uba6, is provided. Compositions and methods for inhibiting ubiquitin via the Uba6 pathway are provided. Methods of identifying novel inhibitors of ubiquitination are also provided. Novel RNAi molecules are also provided.

RELATED APPLICATIONS

This application claims priority from International Application number PCT/US2008/051312, filed Jan. 17, 2008; U.S. provisional patent application No. 60/946,757, filed Jun. 28, 2007; and U.S. provisional patent application No. 60/885,431, filed Jan. 18, 2007; each of which is hereby incorporated herein by reference in its entirety for all purposes.

STATEMENT OF GOVERNMENT INTERESTS

This invention was made with government support under National Institutes of Health grant numbers AG011085 and GM54137. The Government has certain rights in the invention.

BACKGROUND

Protein turnover through the ubiquitin-proteasome pathway (UPP) constitutes a major system used by cells to control signaling networks. In this process, proteins are marked with a chain of ubiquitin molecules, which are linked to the target and to each other through isopeptide linkages with lysine residues (Pickart et al. (2004) Biochim. Biophys. Acta. 1695:55; Pickart (2004) Cell 116:181). Ubiquitin is a 76 amino acid protein which becomes linked to lysine residues through its C-terminal glycine residue. Addition of four or more ubiquitin molecules is generally thought to be sufficient for recognition by the proteasome, where poly-ubiquitinated proteins are degraded (Lam et al. (2002) Nature 416:763). Protein turnover through the UPP is known to regulate many diverse cellular functions and to be intimately linked to human disease (Petroski and Deshaies (2005) Nat. Rev. Mol. Cell. Biol. 6:9). Several components of the ubiquitin system have been found to be mutated in cancer. Most frequently, alterations in the UPP system can lead to inappropriate degradation of tumor suppressor and over-expression of oncogenes, thereby promoting uncontrolled proliferation (Cardozo and Pagano (2004) Nat. Rev. Mol Cell Biol. 5:739). Defects in the UPP can also contribute to neurodegenerative diseases, such as Parkinson's Disease and the like. While ubiquitination plays a critical role in targeted protein degradation, it also plays important roles in controlling protein localization, the function of membrane proteins, endocytosis and other processes.

Protein ubiquitination is known to involve three major protein activities (Hershko et al. (1983) J. Biol. Chem. 258:8206; Ganoth et al. (1988) J. Biol. Chem. 263:12412). First, ubiquitin is activated to form a high-energy thiol ester bond with an E1 ubiquitin activating enzyme. This process initially involves formation of an ubiquitin adenylate with the C-terminal glycine of ubiquitin (G76), consuming one molecule of ATP, followed by transfer of the G76 carboxylate to the active site cysteine to form a thiol ester (Haas et al. (1982) J. Biol. Chem. 257:10329); Haas et al. (1982) J. Biol. Chem. 257:2543). In the second step, the E1˜Ub thiol ester complex binds an E2 ubiquitin conjugating enzyme through the E1s C-terminal E2-binding domain. Ubiquitin is then transferred from the active site cysteine in E1 to the active site cysteine in the E2 (Pickart et al. (1985) J. Biol. Chem. 260:1573. The E2˜Ub complex then dissociates from the E1 where it can interact with E3 ubiquitin ligases to promote transfer of ubiquitin to lysine residues either in the substrate or on growing poly-ubiquitin chains (Eletr et al. (2005) Nat. Struct. Mol. Biol. 12:933). Thus, the E1 enzyme plays a critical role in the ubiquitination process by allowing ubiquitin activation and facilitating ubiquitin transfer.

There is widespread interest in the use of UPP components as drug targets for disease. Indeed, the first drug targeting the UPP, VELCADE® (Millennium Pharmaceuticals, Inc., Cambridge, Mass.), was approved by the FDA in 2003 (Popat et al. (2006) Expert Opin. Pharmacother. 7:1337). VELCADE® targets the 26S proteasome, thereby blocking degradation of all proteins whose turnover requires the proteasome. VELCADE® has proven useful for the treatment of multiple myeloma. Id. Its mode of action appears to rely on the enhanced sensitivity of certain types of cancer cells for proteasome activity, possibly reflecting an increased requirement for the NFκB pathway, which relies on the proteasome. However, proteasome inhibition is also potentially toxic to normal cells and therefore, obtaining new drug targets with increased specificity would be useful.

SUMMARY

The present invention is based in part on the surprising discovery of a novel ubiquitin activating enzyme, Uba6. This discovery is particularly unexpected given the long-standing dogma in the field that a single activating enzyme exists for ubiquitin. The enzymatic properties of Uba6 differ substantially from the classical ubiquitin activating enzyme Ube1, indicating that Uba6 plays biological roles that are distinct from Ube1. Because Uba6 employs ATP in its catalytic mechanism, small molecule inhibitors that selectively inhibit the activity of Uba6 could be useful in controlling signaling pathways that depend specifically upon the activity of Uba6 in organisms such as humans.

In certain embodiments, methods for inhibiting a Uba6 activity are provided. In certain aspects, inhibition is achieved by contacting Uba6 with a compound that inhibits formation of a ubiquitin-adenylate intermediate (e.g., the compound binds an adenylation domain of Uba6), contacting Uba6 with a compound that inhibits thiol esterification of Uba6, or contacting Uba6 with a compound that inhibits transfer of ubiquitin to a ubiquitin conjugating enzyme (e.g., the compound binds a Ub1 domain of Uba6). In certain aspects, inhibition is performed in vitro (e.g., using cell extracts) or in vivo (e.g., in a tissue culture cell or in an organism). In other aspects, the ubiquitin conjugating enzyme is one or more of E2C, E2D1, E2D2, E2D3, E2D4, E2E1, E2G, E2S, E2T and E2Z (also referred to herein as Use1).

In another embodiment, a method for inhibiting ubiquitin activation including contacting Uba6 with a compound that inhibits a catalytic cysteine domain of Uba6 is provided. In certain aspects, inhibition is performed in vitro (e.g., cell extracts) or in vivo (e.g., in a tissue culture cell or in an organism).

In another embodiment, a method of reducing a Uba6 activity in an organism in need thereof including administering to the organism one or more siRNAs complementary to a portion of a Uba6 mRNA is provided. In certain aspects, the siRNA is an RNA sequence including one or more of SEQ ID NO:1, SEQ ID NO:2 and SEQ ID NO:3. In other aspects, the portion of the Uba6 mRNA encodes a ThiF domain, a catalytic cysteine domain, an adenylate domain or a ubiquitin-like domain. In still other aspects, the organism is a human.

In another embodiment, a method of reducing ubiquitination in an organism in need thereof including administering to the organism one or more compounds that inhibits one or more Uba6 activities in the organism is provided. In certain aspects, the compound is an antibody or an siRNA. In other aspects, the organism is a human.

In yet another embodiment, a method of identifying a compound that inhibits charging of E2Z including providing a sample including E2Z and ubiquitin, contacting the sample with the compound, contacting the sample with Uba6 or a biologically active portion thereof, and determining whether the ubiquitin is bound to the E2Z in the presence of the compound, wherein the ubiquitin is not bound to the E2Z if the compound inhibits charging of E2Z is provided. In certain aspects, the method further includes visualizing E2Z on an SDS-PAGE gel.

In still another embodiment, a method of identifying a compound that inhibits a Uba6 activity including providing a sample including a ubiquitin conjugating enzyme and ubiquitin, contacting the sample with the compound, contacting the sample with Uba6 or a biologically active portion thereof, and determining whether the ubiquitin is bound to the ubiquitin conjugating enzyme in the presence of the compound, wherein the ubiquitin is not bound to the ubiquitin conjugating enzyme if the compound inhibits the Uba6 activity is provided. In certain aspects, the ubiquitin conjugating enzyme is selected from the group consisting of: E2C, E2D1, E2D2, E2D3, E2D4, E2E1, E2G, E2S, E2T and E2Z.

In yet another embodiment, a method of identifying a compound that inhibits a Uba6 activity including providing a sample including ubiquitin, contacting the sample with the compound, contacting the sample with Uba6 or a biologically active portion thereof, and determining whether the ubiquitin is bound to the Uba6 or the biologically active portion thereof in the presence of the compound, wherein the ubiquitin is not bound to the Uba6 if the compound inhibits the Uba6 activity is provided. In certain aspects, the ubiquitin is bound to the Uba6 or the biologically active portion thereof via thiol conjugation. In other aspects, the ubiquitin is immobilized.

In another embodiment, an RNA sequence having at least about 70% identity to a nucleic acid sequence selected from the group consisting of SEQ ID NO:1, SEQ ID NO:2 and SEQ ID NO:3, wherein the RNA sequence can inhibit a Uba6 activity is provided. In certain aspects, the Uba6 activity is selected from one or more of ubiquitin activation, ubiquitin-adenylate intermediate formation, ubiquitin thiol esterification, ubiquitin transfer to a ubiquitin-conjugating enzyme and/or ubiquitination of a target polypeptide. In other aspects, the RNA is siRNA.

In another embodiment, an RNA sequence selected from the group consisting of SEQ ID NO:1, SEQ ID NO:2 and SEQ ID NO:3 is provided. In certain aspects, the RNA sequence can inhibit a Uba6 activity such as ubiquitin activation, ubiquitin-adenylate intermediate formation, ubiquitin thiol esterification, ubiquitin transfer to a ubiquitin-conjugating enzyme and/or ubiquitination of a target polypeptide. In other aspects, the RNA is siRNA.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. The foregoing and other features and advantages of the present invention will be more fully understood from the following detailed description of illustrative embodiments taken in conjunction with the accompanying drawings in which:

FIGS. 1A-1H depict physical and functional characteristics of Uba6. (A) Phylogenetic tree for ThiF-domain containing proteins in humans, zebrafish (D. rerio) and sea urchin (S. purpuratus). Also included are mouse Ube1x and Ube1y proteins as well as S. cerevisiae Uba1p. The sequences and tree were generated using ClustalW. (B) SDS-PAGE analysis of Flag-tagged Uba6 (Uba6^(F)) expressed and purified from insect cells (Coomassie staining). (C, D) Uba6 functions as an activating enzyme for ubiquitin but not for several ubiquitin-like proteins (U1ps). The indicated GST-U1ps were purified after expression in bacteria and mixed with purified Uba6F in the presence of ATP. After 10 minutes at 30° C., reaction mixtures were subjected to SDS-PAGE under non-reducing conditions prior to visualization using Coomassie blue. In lane 4, Uba6^(F) is converted to a Uba6^(F)˜GST-Ub conjugate. In panel D, various components were omitted to demonstrate a requirement for GST-Ub and ATP and to demonstrate that the conjugate is sensitive to the reducing agent DTT. *, GST breakdown products. (E) The related E1 enzyme Ube1L promotes activation of ISG15 but not ubiquitin. As an indication of specificity, GST-Ube1L (purified from insect cells) was assayed for activation of GST-ISG15 and GST-Ub. While it was active toward ISG15, as expected, it failed to activate GST-Ub, even though it is more closely related to Ube1 phylogenetically than is Uba6. GST-MP1 was used as a negative control. (F) Kinetics of ubiquitin activation by Ube1 and Uba6 were performed as described further herein. Error bars indicate standard deviation of duplicate assays. (G) Association of Uba6 with ubiquitin-agarose. E1 capture on ubiquitin-Agarose using 293T/Flag-HA-Uba6 extracts (pH 7.5). Extracts from 293T cells stably expressing HA-Flag-Uba6 were subjected to the classical ubiquitin affinity chromatography experiments of Hershko (infra). Extracts were incubated with ubiquitin-agarose in the presence of ATP to allow for ubiquitin charging. Beads were then washed with buffer containing Triton X-100 followed by elution with DTT. Load, flow-thru, washes, eluate and proteins remaining bound to ubiquitin-agarose were then subjected to immunoblotting with anti-Ube1 and anti-HA antibodies to detect HA-Flag-Uba6. (H) Conservation of Uba6 in vertebrates. Alignment of Uba6 and Ube1 from human (Hs), mouse (Mm), S. cerevisiae (Sc) and zebrafish (Danio rerio, Dr). Zebrafish sequences were provided under accession number XP_(—)695755.2. Unlike humans, the mouse contains two closely related (90% identical) Ube1 proteins, Ube1x located on the X-chromosome and Ube1y located on the Y-chromosome. The Ube1y protein is thought to be involved in spermatogenesis (Levy et al. (2000) Mamm. Genome 11:164; Mitchell et al. (1992) Nature 359:528). Dark green, adenylate domain; light green, ThiF motifs; red, catalytic cysteine domain (CCD); blue, ubiquitin-fold domain (Ufd).

FIGS. 2A-2D depict data showing that Uba6 is charged with ubiquitin in vivo. (A) 293T cells stably expressing Flag-HA-Uba6, Flag-HA-Uba6^(C625A), or Flag-HA-Ub6^(C625S) using a lentiviral vector under control of the CMV promoter were lysed in extraction buffer and subjected to immunoprecipitation with anti-HA antibodies. Immune complexes were separated by SDS-PAGE under non-reducing conditions and either immunoblotted with anti-HA, anti-Nedd8, or anti-ubiquitin, or alternatively, the blot was stained with Ponceau S to reveal proteins. (B) 293T cells expressing Flag-HA-Uba6, FLAG-HA-Uba6^(C625A), or Flag-HA-Uba6^(C625S) at near endogenous levels were lysed and extracts subjected to immunoblotting with anti-HA to detect the Flag-HA-Uba6 fusion or with anti-Uba6 to detect both the transgene and the endogenous Uba6 protein. The Flag-HA-Uba6^(C625S) protein migrates as a doublet with the slower mobility band corresponding to the position of ubiquitin-charged Flag-HA-Uba6^(C625S). (C) Identification of ubiquitin in association with Flag-HA-Uba6^(C625S). Extracts from cells expressing Flag-HA-Uba6C625S were subjected to purification using anti-Flag and anti-HA resins and the proteins separated by SDS-PAGE. Individual bands were subjected to mass spectrometry. The slower mobility band contained three peptides from ubiquitin, while the faster mobility form lacked associated ubiquitin-derived peptides. (D) Depletion of Uba6 using RNAi. siRNAs targeting GFP (as a control) or Uba6 were transfected into 293T cells using OLIGOFECTAMINE™ (Invitrogen, Carlsbad, Calif.). After 72 hours, cells were lysed and extracts subjected to immunoblotting using anti-Uba6 antibodies. The siRNA sequences used for Uba6 were: #1: CCUUGGAAGAGAAGCCUGAUGUAAA (SEQ ID NO:1); #2: ACACUGAAGUUAUUGUACCGCAUUU (SEQ ID NO:2); #3: GGGAUCGAUGGACCGUACAUGGAAA (SEQ ID NO:3). Purified Flag-Ha-Uba6 was used as a positive control for the immunoblot.

FIGS. 3A-3C depict data showing that Uba6 and Ube1 display distinct preferences for charging of E2 ubiquitin conjugating enzymes. (A) The indicated E2s were produced using a bacterial in vitro translation system and metabolically labeled with ³⁵S-methionine. E2s were mixed with Flag-HA-Uba6 or Flag-HA-Ube1 (expressed and purified from insect cells) and incubated at 30° C. for 30 minutes. Reaction mixtures were subjected to non-reducing SDS-PAGE and proteins visualized by autoradiography. An aliquot of E2 was included as a control for charging. (B) Summary of E2 charging data. (C) The ability of Uba6 to promote charging of E2D1 requires its active site cysteine (C625).

FIGS. 4A-4C depict data showing that E2 specificity of Uba6 resides in its C-terminal Ub1 domain. (A) Domain structures of Ube1 and Uba6 showing the percent conservation over the catalytic domains and the C-terminal Ubiquitin-like (Ub1) domain. (B) Conservation of the C-terminal Ub1 domains from Ube1 and Uba6. Alignment was generated using ClustalW. (C) Chimeric proteins in which the Ub1 domains of Uba6 and Ube1 were swapped with each other were created. These proteins, along with wild-type and Ub1-deletion mutants, were expressed in insect cells as Flag-HA fusion proteins, purified, and assayed for their ability to promote charging of Cdc34. Cdc34 is charged by Ube1 but not Uba6 (lanes 2 and 3). Both E1s lacking the Ub1 domain were inactive, while Uba6 containing the Ub1 domain of Ube1 was active for Cdc34 charging.

FIG. 5 depicts the cDNA sequence encoding Homo sapiens Uba6 (SEQ ID NO:4) (GenBank Accession Number NM_(—)018227).

FIG. 6 depicts the amino acid sequence of Homo sapiens Uba6 (SEQ ID NO:5) (GenPept Accession Number NP_(—)060697).

FIG. 7 depicts the cDNA sequence encoding Mus musculus Uba6 (SEQ ID NO:6) (Gen Bank Accession Number NM_(—)172712).

FIG. 8 depicts the amino acid sequence of Mus musculus Uba6 (SEQ ID NO:7) (GenPept Accession Number NP_(—)766300).

FIG. 9 depicts the cDNA sequence encoding E2Z from Homo sapiens (SEQ ID NO:8) (Gen Bank Accession Number NM_(—)023079.2).

FIG. 10 depicts the amino acid sequence encoding E2Z from Homo sapiens (SEQ ID NO:9) (GenPept Accession Number NP_(—)075567.1 has the partial sequence).

FIGS. 11A-11D depict data showing that the C-terminal domains of Ube1 and Uba6 control E2 specificity. (A) Domain structures of Uba6 and Ube1 and a schematic showing how the C-terminal Ub1 domains were interchanged to form the chimeric proteins. (B) Model of the domain structure of an E1 enzyme based on the crystal structure of the Nedd8 activating enzyme. (C) E2 charging assays were performed using the indicated E1 proteins and E2 conjugating enzymes. (D) Predicted structures of the Ub1 domains of Ube1 and Uba6.

FIG. 12 depicts data showing that Uba6 is required to charge E2Z in vivo. Left panel: formation of the E2Z-ubiquitin thiolester required the active site cysteine of E2Z (C>A mutant) and was reduced by addition of DTT. Right panel: Depletion of Uba6, but not Ube1, by RNAi blocked charging of E2Z in vivo. The indicated siRNAs were transfected into 293T cells stably expressing Flag-HA-E2Z. Extracts were generated in MES buffer (pH 4.5) and proteins immediately separated by SDS-PAGE. After transfer to nitrocellulose, blots were probed with the indicated antibodies.

FIGS. 13A-E depict Uba6 activation of ubiquitin in vivo. (A) E1 expression in cultured cells. Anti-Uba6 and anti-Ube1 were used to probe blots of extracts from the indicated cell lines, with recombinant Flag-E1a as controls. (B) Lysates (pH 7.5) from 293T/Flag-HA-Uba6 cells (wild-type, C625S or C625A; 20 μg) or anti-HA immune complexes from 0.2 mg of extract were separated on 4-12% Tris-glycine reducing gels and immunoblotted with anti-Uba6, anti-HA or anti-ubiquitin. (C) Lysates (pH 4.5) from 293T/Flag-HA-Uba6 cells (wild-type or C625A; 20 μg) or anti-HA immune complexes from 0.2 mg of extract were separated on 4-12% Bis-Tris non-reducing gels prior to immunoblotting with anti-HA or anti-ubiquitin. (D) Flag-HA-Uba6 was immunoprecipitated from 293T/Flag-HA-Uba6 cell lysates (pH 4.5, 2 mg) and separated on a 4-12% Bis-Tris non-reducing gel. (E) Mass spectral analysis of ubiquitin-activated Uba6. The Flag-HA-Uba6^(C625S) protein used is described further herein.

FIGS. 14A-14D depict the systematic analysis of E2 conjugating enzymes for targets of Uba6. (A) E2 charging activity of ubiquitin E1s depicted on a phylogenetic tree of active E2s. (B) Uba6 and Ube1 display distinct E2 charging activities in vitro. Assays employed ³⁵S-methionine labeled E2s made in E. coli S30 extracts, KO ubiquitin, and the indicated E1, as described in METHODS. *, non-specific translation products. (C) Sequence conservation of human Ube1 and Uba6. (D) Charging of Cdc34B, UbCH5D, and Use1 (also referred to herein as E2Z) in vitro by chimeric E1 proteins was examined using ³⁵S-methionine labeled E2s.

FIGS. 15A-15D depict the distinct requirements for charging of the ubiquitin conjugating enzymes Use1 and Cdc34 in vivo. (A) Scheme depicting the mechanism of E2 charging in cells. E2s exist as a mixture of charged and uncharged forms, depending upon how rapidly the charged form is generated and used. (B) HeLa cells were transfected with the indicated siRNAs. After 72 hours, cells were lysed at pH 4.5, proteins separated on a non-reducing 4-12% Bis-Tris gradient gel and immunoblotted. (C) Use1 is charged in mammalian cells through its catalytic cysteine. 293T or 293T/Flag-HA-Use1 (wild-type or C190A) cells were lysed (pH 4.5) prior to separation on a non-reducing 4-12% Bis-Tris gradient gel and immunoblotting. In lane 4, the sample was pre-treated with DTT (200 mM, 5 minutes). (D) A model depicting two independent systems for activating and charging ubiquitin.

FIGS. 16A-16C depict structural information of E1s. (A) Structural organization of E1s. Ube1, UbeL1 and Uba6 are single chain E1s while Uba3/APP-BP1 and Uba2/Sae1 are heterodimeric E1s. (B) Predicted structure of Ufd domains from Uba6 and Ube1, and a comparison with the Ufd domain from Uba3. Predicted structures were generated using “Modeller” software (release 8v2) with the SUMO E1 Sae2 as template (PDB code 1Y8Q) and displayed using Pymol. Ufd^(Ube1): residues 948-1058. Ufd^(Uba6): residues 949-1052. Ufd^(Uba3): residues 349-440. (C) Domain specific sequence identities among human Uba6, Ube1 and Ube1L. Percent identities and percent similarities are shown.

FIG. 17 depicts that Uba6 and Use1 are widely expressed in human cell lines and tissues. Expression of Ube1 is shown for comparison. Expression patterns were obtained using the Genomics Institute of the Novartis Research Foundation transcriptional profiling resource (Su et al. (2002) Proc. Natl. Acad. Sci. U.S.A. 99:4465; Su et al. (2004) Proc. Natl. Acad. Sci. U.S.A. 101:6062).

FIGS. 18A-18F depict the analysis of GST-UBL proteins. (A) Ube1L activates ISG15 but not ubiquitin. GST-Ube1L, GST-Ube1, or GST-MP1 as a negative control (300 nM) were tested for their ability to activate GST-ISG15 or GST-Ub (6 μM) in the presence of 2 mM ATP (30 min, 30° C.). Reaction mixtures were subjected to 4-12% Tris-glycine and proteins visualized with Coomassie Blue. GST-Ube1L, GST-Ube1, and GST-MP1 were expressed in insect cells and purified as described previously (Zhao et al. (2004) Proc. Natl. Acad. Sci. U.S.A. 101:7578). (B) GST-Nedd8 is activated by Uba3/APPBP1 in vitro. Uba3/APPBP1 (300 nM) was incubated with 6 μM GST-Nedd8 or GST as a control (as described in panel a) and analyzed for thioester formation by non-reducing 4-12% Tris-glycine. (C) Uba6 does not activate Nedd8 in vitro. Flag-Uba6 (8 nM) was incubated with varying concentrations of GST-Ub or GST-Nedd8 (0, 5, 10, 20, 40, 80 ng) in the presence of 2 mM ATP and reaction mixtures subjected to 4-12% Tris-glycine and immunoblotting using anti-Flag antibodies. Uba6 activated GST-Ub but not GST-Nedd8. (D) GST-ubiquitin titration. Eight nM Uba6 or Ube1 was incubated with the indicated concentration of GST-ubiquitin for 10 min at 30° C. Samples were subjected to electrophoresis on a 4-12% Tris-glycine non-reducing gel prior to immunoblotting with the indicated antibody. (E) Time course for GST-ubiquitin activation. Reactions contained 500 nM GST-ubiquitin and 8 nM Uba6 or Ube1 and were incubated for the indicated time at 30° C. (F) Summary of mass spectral data identifying C-terminal peptides from GST-UBL proteins used in UBL activation assays.

FIGS. 19A-19B depict that Uba6 activates ubiquitin but not Nedd8 in vivo and in vitro. (A) Tandem mass spectra for a ubiquitin-derived peptide co-migrating with the slow-mobility form of Flag-HA-Uba6^(C625S) isolated from 293T cells. The spectra of the TLSDYNIQK peptide from ubiquitin is shown. (B) Flag-HA-Uba6 was isolated from extracts (10 mg) derived from the indicated cell lines lysed in pH 7.5 buffer and proteins separated by non-reducing 4-12% Tris-glycine followed by staining with Coomassie Blue. Upper and lower bands were subjected to mass spectrometer. The ubiquitin peptides identified are shown and the relevant statistics for these peptides were collected as described further herein.

FIGS. 20A-20G depict sequence alignments of Use1 proteins from vertebrates and analysis of E2 charging. (A) Phylogenetic tree of the human E2 ubiquitin conjugating enzyme family, including the non-catalytic sub-class typified by Uev. The accession numbers for individual E2 family members used to generate the tree are provided. E2 sequences were aligned in ClustalW and the tree displayed in Treeview. (B) Alignments were generated in Clustal W. Sequences for frog (Xenopus) and zebrafish (Danio rerio) Use1 were obtained by blast analysis of the EST database using human Use1 as the query. Mm gene ID: 268470; Rn gene ID: 303478; Hs gene ID: 65264; Danio accession number: XP_(—)001337511; Xenopus accession number: Q6PCF7; Spur (S. purpuratus) accession number: XP_(—)785296. (C) The catalytic cysteine of Uba6 is required for charging of Ubc5 and Use1. Assays were performed as described further herein using wild-type or mutant (C625A) Uba6. (D) Charging of purified Use1 by Uba6. Use1 was purified from bacteria as described under METHODS. Use1 (2.5 μM) was used in charging assays with increasing concentrations of Flag-Uba6 (0, 0.025, 0.05, 0.1, 0.2, 0.4 μM, purified from insect cells) in the presence of 25 μM ubiquitin and 2 mM ATP. (E) Amino acid sequences of the Ufd domains from Ube1 and Uba6 proteins. Black and grey, conserved in Ube1 and Uba6; purple, conserved in Ube1; yellow, conserved in Uba6. (F) An assay that measures the extent of charging for ubiquitin E2s. Cells were lysed in buffers from pH 3 to pH 5 as described further herein, subjected to non-reducing 4-12% Bis-Tris gel, and immunoblots probed with anti-Cdc34, which reacts with both Cdc34A and Cdc34B proteins. In lanes 5-8, lysates were boiled in the presence of 200 mM DTT for 5 min prior to electrophoresis. (G) Specificity of Use1 antibodies. 293T cell extracts for cells transfected with the indicated siRNAs (72 hours post-transfection) were subjected to 4-12% Tris-glycine under reducing conditions and immunoblotted with affinity purified Use1 antibody. (H) 293T cells were lysed in pH 4.5 MES Lysis buffer and 10 μg subjected to electrophoresis on a 4-12% Bis-Tris gel in the presence of absence of DTT. Gels were probed against the indicated antibodies.

FIGS. 21A-21E depict structural modeling of Uba6Ufd-Use1 and Ube1^(Ufd)-Cdc34 interfaces. (A) Model-based sequence alignments of human Uba6, Ube1, and Uba3 as well as alignments for the H1 helices of Cdc34A, Use1, and Ubc12. Residues located at the interface are shown in red. Identical residues are shaded in black. Similar residues are shaded in grey. (B) Structure of the Ubc12-Uba3^(Ufd) (Huang et al. (2005) Mol. Cell. 17:341). Ubc12 is in marine. Uba3^(Ufd) is in green. Residues at the interface are indicated. (C) Model of Use1-Uba6^(Ufd). Use1 is in marine. Uba6^(Ufd) is in green. Residues at the interface are indicated. (D) Model of Cdc34AUbe1^(Ufd). Cdc34A is in marine. Ube1^(Ufd) is in green. Residues at the interface are indicated. (E) Surface depiction of charged residues in Uba3^(Ufd) (structure), Uba6^(Ufd) (model), and Ube1^(Ufd) (model). The corresponding ribbon images are shown for orientation purposes.

FIG. 22 depicts the sequences of genes examined as described further herein.

DETAILED DESCRIPTION

It is generally thought that Ube1 is the sole activating enzyme for ubiquitin encoded by the genomes of all eukaryotes. This conclusion is based on the presence of a single essential ubiquitin E1 in budding yeast and the finding that rodent tissue culture cells containing temperature sensitive mutations in Ube1 display cell cycle arrest phenotypes consistent with there being only a single gene (Finley et al. (1984) Cell 37:43; Ciechanover et al. (1984) J. Cell. Biochem. 24:27). As described further herein, it has surprisingly been discovered that this dogma is incorrect, and vertebrate organisms contain two distinct activating enzymes for ubiquitin. Without intending to be bound by theory, this novel enzyme, Uba6, promotes the conjugation of ubiquitin to ubiquitin conjugating enzymes (e.g., E2s) in a manner that appears to be distinct from Ube1. Accordingly, Uba6 can have specific functions in the ubiquitin pathway and, therefore, is an important drug target.

The ubiquitin-proteasome pathway (UPP) plays a central role in the turnover of many key regulatory proteins involved in transcription, cell cycle progression and apoptosis, all of which are important in disease states. See, e.g., King et al. (1996) Science 274:1652; Vorhees et al. (2003) Clin. Cancer Res. 9:6316; Adams et al. (2004) Nat. Rev. Cancer 4:349. Accordingly, targeting ubiquitin-activating enzymes provides a unique opportunity to interfere with a variety of biochemical pathways important for maintaining the integrity of cell division and cell signaling. Ubiquitin-activating enzymes, such as Uba6, function at the first step of ubiquitin conjugation pathways. Thus, inhibition of a Uba6 enzyme should specifically modulate the downstream biological consequences of a ubiquitin modification. As such, inhibition of these activating enzymes, and the resultant inhibition of downstream effects of ubiquitin-conjugation, represents a method of interfering with the integrity of cell division, cell signaling, and several aspects of cellular physiology which are important for disease mechanisms. Thus, ubiquitin-activating enzymes such as Uba6, as regulators of diverse cellular functions, are potentially important therapeutic targets for the identification of novel approaches to treatment of a variety of diseases and disorders (i.e., “ubiquitin-related disorders”).

In at least certain embodiments, the Uba6 and/or E2Z (also referred to herein as Use1) modulating agents described herein can be used in the treatment of cellular proliferative disorders, e.g., cancer. The role of the UPP pathway in oncogenesis has led to the investigation of proteasome inhibition as a potential anticancer therapy. For example, modulation of the UPP pathway by inhibition of the 26S proteasome by VELCADE™ has proven to be an effective treatment in certain cancers and is approved for the treatment of relapsed and refractory multiple myeloma.

Examples of proteins whose levels are controlled by the UPP pathway include the CDK inhibitor p27^(Kip1) and the inhibitor of NFκB, IκB. See, Podust et al. (2000) Proc. Natl. Acad. Sci. USA 97:4579; Read et al. (2000) Mol. Cell. Biol. 20:2326. Inhibition of the degradation of p27 can block the progression of cells through the G1 and S phases of the cell cycle. Interfering with the degradation of IκB can prevent the nuclear localization of NF-κB, transcription of various NF-κB-dependent genes associated with the malignant phenotype, and resistance to standard cytotoxic therapies. NF-κB plays a key role in the expression of a number of pro-inflammatory mediators, implicating a role for such inhibitors in inflammatory disorders. Accordingly, inhibition of UPP is useful for the treatment of inflammatory disorders, including, e.g., rheumatoid arthritis, asthma, multiple sclerosis, psoriasis, reperfusion injury and the like.

Inhibition of UPP is also useful for treatment of disorders such as neurodegenerative disorders, including e.g., Parkinson's disease, Alzheimer's disease, triplet repeat disorders; neuropathic pain; ischemic disorders, e.g., stroke, infarction, kidney disorders; and cachexia. See, e.g., Elliott and Ross (2001) Am. J. Clin. Pathol. 116:637; Elliott et al. (2003) J. Mol. Med. 81:235; Tarlac and Storey (2003) J. Neurosci. Res. 74:406; Mori et al. (2005) Neuropath. Appl. Neurobiol. 31:53; Manning (2004) Curr. Pain Headache Rep. 8:192; Dawson and Dawson (2003) Science 302:819; Kukan (2004) Physiol. Pharmacol. 55:3; Wojcik and DiNapoli (2004) Stroke 35:1506; Lazarus et al. (1999) Am. J. Physiol. 27:E332.

Treatment of cellular proliferative disorders is intended to include inhibition of proliferation including rapid proliferation. As used herein, the term “cellular proliferative disorder” includes disorders characterized by undesirable or inappropriate proliferation of one or more subset(s) of cells in a multicellular organism. The term “cancer” refers to various types of malignant neoplasms, most of which can invade surrounding tissues, and may metastasize to different sites (see, for example, PDR Medical Dictionary 1st edition (1995)). The terms “neoplasm” and “tumor” refer to an abnormal tissue that grows by cellular proliferation more rapidly than normal and continues to grow after the stimuli that initiated proliferation is removed (see, for example, PDR Medical Dictionary 1st edition (1995)). Such abnormal tissue shows partial or complete lack of structural organization and functional coordination with the normal tissue which may be either benign (i.e., benign tumor) or malignant (i.e., malignant tumor).

The language “treatment of cellular proliferative disorders” is intended to include the prevention of the growth of neoplasms in a subject or a reduction in the growth of pre-existing cancers in a subject. The inhibition also can be the inhibition of the metastasis of a cancer from one site to another.

In certain embodiments, the cancer is a solid tumor. Non-limiting examples of solid tumors that can be treated by the methods of the invention include pancreatic cancer; bladder cancer; colorectal cancer; breast cancer, including metastatic breast cancer; prostate cancer, including androgen-dependent and androgen-independent prostate cancer; renal cancer, including, e.g., metastatic renal cell carcinoma; hepatocellular cancer; lung cancer, including, e.g., non-small cell lung cancer (NSCLC), bronchioloalveolar carcinoma (BAC), and adenocarcinoma of the lung; ovarian cancer, including, e.g., progressive epithelial or primary peritoneal cancer; cervical cancer; gastric cancer; esophageal cancer; head and neck cancer, including, e.g., squamous cell carcinoma of the head and neck; melanoma; neuroendocrine cancer, including metastatic neuroendocrine tumors; brain tumors, including, e.g., glioma, anaplastic oligodendroglioma, adult glioblastoma multiforme, and adult anaplastic astrocytoma; bone cancer; soft tissue sarcoma and the like.

In certain other embodiments, the cancer is a hematologic malignancy. Non-limiting examples of hematologic malignancy include acute myeloid leukemia (AML); chronic myelogenous leukemia (CML), including accelerated CML and CML blast phase (CML-BP); acute lymphoblastic leukemia (ALL); chronic lymphocytic leukemia (CLL); Hodgkin's disease (HD); non-Hodgkin's lymphoma (NHL), including follicular lymphoma and mantle cell lymphoma; B-cell lymphoma; T-cell lymphoma; multiple myeloma (MM); Waldenstrom's macroglobulinemia; myelodysplastic syndromes (MDS), including refractory anemia (RA), refractory anemia with ringed siderblasts (RARS), (refractory anemia with excess blasts (RAEB), and RAEB in transformation (RAEB-T); myeloproliferative syndromes and the like.

Cellular proliferative disorders can further include disorders associated with hyperproliferation of vascular smooth muscle cells such as proliferative cardiovascular disorders, e.g., atherosclerosis and restinosis. Cellular proliferation disorders can also include disorders such as proliferative skin disorders, e.g., X-linked ichthyosis, psoriasis, atopic dermatitis, allergic contact dermatitis, epidermolytic hyperkeratosis, and seborrheic dermatitis. Cellular proliferative disorders can further include disorders such as autosomal dominant polycystic kidney disease (ADPKD), mastocystosis, and cellular proliferation disorders caused by infectious agents such as viruses.

Pharmaceutical Compounds

Compounds of the present invention include inhibitors of one or more ubiquitin-activating enzyme activities. In particular, the compounds are designed to be inhibitors of one or more Uba6 activities (e.g., ubiquitin activation; ubiquitin-adenylate intermediate formation; ubiquitin thiol esterification; ubiquitin transfer to one or more ubiquitin-conjugating enzymes (e.g., E2s); and/or ubiquitination of one or more target polypeptides (e.g., ubiquitin, an E2, a target polypeptide and/or protein or the like)) and/or one or more E2Z activities (e.g., ubiquitin activation; ubiquitin-adenylate intermediate formation; ubiquitin thiol esterification; ubiquitin transfer to one or more ubiquitin-protein ligases (e.g., E3s); and/or ubiquitination of one or more target polypeptides (e.g., ubiquitin, an E3, a target polypeptide and/or protein or the like)).

Inhibitors include, but are not limited to, compounds which modulate (e.g., reduce) the promoting effects of E1 enzymes in ubiquitin conjugation to target proteins (e.g., reduction of ubiquitin activation and/or facilitating ubiquitin transfer to E2). Inhibitors also include, but are not limited to, compounds which modulate (e.g., reduce) the promoting effects of E2Z enzymes in ubiquitin conjugation to target proteins (e.g., reduction of ubiquitin activation and/or facilitating ubiquitin transfer to E3). Thus, the compounds of this invention may be assayed for their ability to inhibit a Uba6 enzyme and/or an E2Z enzyme in vitro or in vivo (e.g., in cells or animal models) according to methods provided in further detail herein, or methods known in the art. The compounds may be assessed for their ability to bind or mediate a Uba6 enzyme and/or an E2Z enzyme activity directly. Alternatively, the activity of compounds may be assessed through indirect cellular assays, or assays of downstream effects of Uba6 and/or E2Z activation to assess inhibition of downstream effects of Uba6 inhibition. For example, activity may be assessed by detection of ubiquitin-conjugated substrates (e.g., ubiquitin-conjugated E2s, ubiquitin-conjugated E3s, ubiquitinated substrates and the like); detection of downstream protein substrate stabilization; detection of inhibition of UPP activity; and the like. Assays for assessing activities are described further herein and/or are known in the art.

One embodiment of this invention relates to a composition comprising a compound described herein or a pharmaceutically acceptable salt thereof, and a pharmaceutically acceptable carrier. It will be appreciated that the compounds of this invention may be derivatized at functional groups to provide prodrug derivatives which are capable of conversion back to the parent compounds in vivo. Examples of such prodrugs include the physiologically acceptable and metabolically labile ester derivatives, such as methoxymethyl esters, methylthiomethyl esters, or pivaloyloxymethyl esters derived from a hydroxyl group of the compound or a carbamoyl moiety derived from an amino group of the compound. Additionally, any physiologically acceptable equivalents of the present compounds, similar to the metabolically labile esters or carbamates, which are capable of producing the parent compounds described herein in vivo, are within the scope of this invention.

If pharmaceutically acceptable salts of the compounds of the invention are utilized in these compositions, the salts may be derived from inorganic or organic acids and bases. For reviews of suitable salts, see, e.g., Berge et al (1977) J. Pharm. Sci. 66:1; and Remington: The Science and Practice of Pharmacy, 20th Ed., ed. A. Gennaro, Lippincott Williams & Wilkins, 2000.

As used herein, the term “pharmaceutically acceptable carrier” is intended to include, but is not limited to, a material that is compatible with a recipient subject, such as a mammal (e.g., a human), and is suitable for delivering an active agent to the target site without terminating the activity of the agent. The toxicity or adverse effects, if any, associated with the carrier preferably are commensurate with a reasonable risk/benefit ratio for the intended use of the active agent.

The pharmaceutical compositions of the invention can be manufactured by methods well known in the art such as conventional granulating, mixing, dissolving, encapsulating, lyophilizing, or emulsifying processes, among others. Compositions may be produced in various forms, including granules, precipitates, or particulates, powders, including freeze dried, rotary dried or spray dried powders, amorphous powders, tablets, capsules, syrup, suppositories, injections, emulsions, elixirs, suspensions or solutions. Formulations may optionally contain stabilizers, pH modifiers, surfactants, solubilizing agents, bioavailability modifiers and combinations of these.

Pharmaceutical formulations may be prepared as liquid suspensions or solutions using a liquid, such as, but not limited to, an oil, water, an alcohol, and combinations of these. Solubilizing agents such as cyclodextrins may be included. Pharmaceutically suitable surfactants, suspending agents, or emulsifying agents, may be added for oral or parenteral administration. Suspensions may include oils, e.g., peanut oil, sesame oil, cottonseed oil, corn oil, olive oil and the like. Suspension preparations may also contain esters of fatty acids such as ethyl oleate, isopropyl myristate, fatty acid glycerides and acetylated fatty acid glycerides. Suspension formulations may include alcohols including, but not limited to, ethanol, isopropyl alcohol, hexadecyl alcohol, glycerol and propylene glycol. Ethers, including, but not limited to, poly(ethyleneglycol), petroleum hydrocarbons such as mineral oil and petrolatum; and water may also be used in suspension formulations.

Pharmaceutically acceptable carriers that may be used in these compositions include, but are not limited to, ion exchangers, alumina, aluminum stearate, lecithin, serum proteins, such as human serum albumin, buffer substances such as phosphates and carbonates, glycine, sorbic acid, potassium sorbate, partial glyceride mixtures of saturated vegetable fatty acids, water, salts or electrolytes, such as protamine sulfate, disodium hydrogen phosphate, potassium hydrogen phosphate, sodium chloride, zinc salts, colloidal silica, magnesium trisilicate, polyvinyl pyrrolidone, cellulose-based substances, polyethylene glycol, sodium carboxymethylcellulose, polyacrylates, waxes, polyethylene-polyoxypropylene-block polymers, polyethylene glycol and wool fat.

According to certain embodiments, the compositions of this invention are formulated for pharmaceutical administration to a mammal, such as a human. Such pharmaceutical compositions may be administered orally, parenterally, by inhalation spray, topically, rectally, nasally, buccally, vaginally or via an implanted reservoir. The term “parenteral,” as used herein includes, but is not limited to, subcutaneous, intravenous, intraperitoneal, intramuscular, intra-articular, intra-synovial, intrasternal, intrathecal, intrahepatic, intralesional and intracranial injection or infusion techniques. In certain aspects, the compositions are administered orally, intravenously, or subcutaneously. The formulations of the invention may be designed to be short-acting, fast-releasing, or long-acting. Still further, compounds can be administered in a local rather than systemic means, such as administration (e.g., by injection) at a tumor site.

Sterile, injectable forms of the compositions described herein may be aqueous or oleaginous suspensions. These suspensions may be formulated according to techniques known in the art using suitable dispersing or wetting agents and suspending agents. The sterile injectable preparation may also be a sterile injectable solution or suspension in a non-toxic parenterally acceptable diluent or solvent, for example as a solution in 1,3-butanediol. Among the acceptable vehicles and solvents that may be employed are water, Ringer's solution and isotonic sodium chloride solution. In addition, sterile, fixed oils are conventionally employed as a solvent or suspending medium. For this purpose, any bland fixed oil may be employed including synthetic mono- or di-glycerides. Fatty acids, such as oleic acid and its glyceride derivatives are useful in the preparation of injectables, as are natural pharmaceutically-acceptable oils, such as olive oil or castor oil, especially in their polyoxyethylated versions. These oil solutions or suspensions may also contain a long-chain alcohol diluent or dispersant, such as carboxymethyl cellulose or similar dispersing agents which are commonly used in the formulation of pharmaceutically acceptable dosage forms including emulsions and suspensions. Other commonly used surfactants, such as Tweens, and other emulsifying agents or bioavailability enhancers which are commonly used in the manufacture of pharmaceutically acceptable solid, liquid, or other dosage forms may also be used for the purposes of formulation. Compounds may be formulated for parenteral administration by injection such as by bolus injection or continuous infusion. A unit dosage form for injection may be in ampoules or in multi-dose containers.

Pharmaceutical compositions may be orally administered in any orally acceptable dosage form including, but not limited to, capsules, tablets, aqueous suspensions or solutions. In the case of tablets for oral use, carriers that are commonly used include lactose and corn starch. Lubricating agents, such as magnesium stearate, are also typically added. For oral administration in a capsule form, useful diluents include lactose and dried cornstarch. When aqueous suspensions are required for oral use, the active ingredient is combined with emulsifying and suspending agents. If desired, certain sweetening, flavoring or coloring agents may also be added.

Alternatively, pharmaceutical compositions may be administered in the form of suppositories for rectal administration. These may be prepared by mixing the agent with a suitable non-irritating excipient which is solid at room temperature but liquid at rectal temperature and therefore will melt in the rectum to release the drug. Such materials include cocoa butter, beeswax and polyethylene glycols.

Pharmaceutical compositions of this invention may also be administered topically, especially when the target of treatment includes areas or organs readily accessible by topical application, including diseases of the eye, the skin, or the lower intestinal tract. Suitable topical formulations are readily prepared for each of these areas or organs.

Topical application for the lower intestinal tract may be effected in a rectal suppository formulation (see above) or in a suitable enema formulation. Topically-transdermal patches may also be used. For topical applications, the pharmaceutical compositions may be formulated in a suitable ointment containing the active component suspended or dissolved in one or more carriers. Carriers for topical administration of compounds described herein include, but are not limited to, mineral oil, liquid petrolatum, white petrolatum, propylene glycol, polyoxyethylene, polyoxypropylene compound, emulsifying wax and water. Alternatively, the pharmaceutical compositions may be formulated in a suitable lotion or cream containing the active components suspended or dissolved in one or more pharmaceutically acceptable carriers. Suitable carriers include, but are not limited to, mineral oil, sorbitan monostearate, polysorbate 60, cetyl esters wax, cetearyl alcohol, 2-octyldodecanol, benzyl alcohol and water.

For ophthalmic use, the pharmaceutical compositions may be formulated as micronized suspensions in isotonic, pH adjusted sterile saline, or, preferably, as solutions in isotonic, pH adjusted sterile saline, either with our without a preservative such as benzylalkonium chloride. Alternatively, for ophthalmic uses, the pharmaceutical compositions may be formulated in an ointment such as petrolatum.

Pharmaceutical compositions may also be administered by nasal aerosol or inhalation. Such compositions are prepared according to techniques well known in the art of pharmaceutical formulation and may be prepared as solutions in saline, employing benzyl alcohol or other suitable preservatives, absorption promoters to enhance bioavailability, fluorocarbons, and/or other conventional solubilizing or dispersing agents.

Pharmaceutical compositions described herein are particularly useful in therapeutic applications relating to disorders as described herein (e.g., proliferation disorders, e.g., cancers, inflammatory disorders, neurodegenerative disorders). In certain embodiments, the composition is formulated for administration to a patient having or at risk of developing or experiencing a recurrence of the relevant disorder being treated. The term “patient,” as used herein, is intended to refer to an animal, e.g., a mammal such as a human. In certain embodiments, a pharmaceutical composition described herein may further comprise another therapeutic agent. The therapeutic agent can be one normally administered to patients with the disorder, disease or condition being treated.

By “therapeutically effective amount” is meant an amount of compound or composition sufficient, upon single or multiple dose administration, to cause a detectable decrease in one or more Uba6 enzyme activities and/or the severity of one or more disorders or disease states being treated. “Therapeutically effective amount” is also intended to include an amount sufficient to treat a cell, prolong or prevent advancement of the disorder or disease state being treated (e.g., prevent additional tumor growth of a cancer, prevent additional inflammatory response), ameliorate, alleviate, relieve, or improve a subject's symptoms of the a disorder beyond that expected in the absence of such treatment.

The amount of a Uba6 enzyme modulator (e.g., inhibitor) required will depend on the particular compound of the composition given, the type of disorder being treated, the route of administration, and the length of time required to treat the disorder. A therapeutically effective amount of compound (i.e., an effective dosage) can range from about 0.001 to 30 mg/kg body weight, about 0.01 to 25 mg/kg body weight, about 0.1 to 20 mg/kg body weight, about 1 to 10 mg/kg, 2 to 9 mg/kg, 3 to 8 mg/kg, 4 to 7 mg/kg, or 5 to 6 mg/kg body weight. It should also be understood that a specific dosage and treatment regimen for any particular patient will depend upon a variety of factors, including the activity of the specific compound employed, the age, body weight, general health, sex, and diet of the patient, time of administration, rate of excretion, drug combinations, the judgment of the treating physician, and the severity of the particular disease being treated. In certain aspects where the inhibitor is administered in combination with another agent, the amount of additional therapeutic agent present in a composition typically will be no more than the amount that would normally be administered in a composition comprising that therapeutic agent as the only active agent. In certain aspects, the amount of additional therapeutic agent will range from about 50% to about 100% of the amount normally present in a composition comprising that agent as the only therapeutically active agent.

One embodiment of the invention relates to a method of modulating (e.g., inhibiting) or decreasing one or more Uba6 enzyme activities in a sample comprising contacting the sample with one or more compounds and/or compositions described herein. The sample can include, without limitation, a purified or partially purified Uba6 enzyme, cultured cells or extracts of cell cultures, biopsied cells or fluid obtained from a mammal or extracts thereof, and one or more bodily fluids (e.g., blood, serum, saliva, urine, feces, semen, tears, breast milk and the like) or extracts thereof. Inhibition of one or more Uba6 enzyme activities in a sample may be carried out in vitro, in vivo or in situ.

In another embodiment, method for treating a patient having one or more ubiquitin-related disorders described herein, one or more symptoms of one or more ubiquitin-related disorders described herein, and/or at risk of developing or experiencing a recurrence of one or more ubiquitin-related disorders described herein. Such methods comprise administering to the patient one or more compounds and/or pharmaceutical composition described herein. As used herein, the term “treating” is intended to include, but is not limited to, curing, healing, alleviating, relieving, altering, remedying, ameliorating, palliating, improving or affecting the disorder, one or more symptoms of the disorder or the predisposition toward the disorder.

Depending on the particular disorder or condition to be treated, one or more Uba6 enzyme inhibitors are administered in conjunction with additional therapeutic agent or agents. In certain embodiments, the additional therapeutic agent is one that is normally administered to patients with the disorder or condition being treated. The one or more Uba6 inhibitors may be administered with the other therapeutic agent in a single dosage form or as a separate dosage form. When administered as a separate dosage form, the other therapeutic agent may be administered prior to, at the same time as, or following administration of the one or more Uba6 inhibitors.

In certain embodiments, the one or more Uba6 inhibitors are administered in conjunction with one or more therapeutic agents including, but not limited to, cytotoxic agents, radiotherapy, and immunotherapy appropriate for treatment of proliferative disorders and cancer. Non-limiting examples of cytotoxic agents suitable for use in combination with the one or more Uba6 inhibitors include antimetabolites (e.g., capecitibine, gemcitabine, 5-fluorouracil or 5-fluorouracil/leucovorin, fludarabine, cytarabine, mercaptopurine, thioguanine, pentostatin, and methotrexate); topoisomerase inhibitors (e.g., etoposide, teniposide, camptothecin, topotecan, irinotecan, doxorubicin, and daunorubicin); vinca alkaloids (e.g., vincristine and vinblastin; taxanes, including, e.g., paclitaxel and docetaxel); platinum agents (e.g., cisplatin, carboplatin, and oxaliplatin); antibiotics (e.g., actinomycin D, bleomycin, mitomycin C, adriamycin, daunorubicin, idarubicin, doxorubicin and pegylated liposomal doxorubicin); alkylating agents (e.g., melphalan, chlorambucil, busulfan, thiotepa, ifosfamide, carmustine, lomustine, semustine, streptozocin, decarbazine, and cyclophosphamide); protein tyrosine kinase inhibitors (e.g., imatinib mesylate and gefitinib); proteasome inhibitors (e.g., bortezomib, thalidomide and related analogs); antibodies (e.g., trastuzumab, rituximab, cetuximab, and bevacizumab); mitoxantrone; dexamethasone; prednisone; temozolomide and the like.

Other examples of agents the one or more Uba6 inhibitors may be combined with include anti-inflammatory agents (e.g., corticosteroids, TNF blockers, Il-1 RA, azathioprine, cyclophosphamide, and sulfasalazine); immunomodulatory and/or immunosuppressive agents (e.g., cyclosporine, tacrolimus, rapamycin, mycophenolate mofetil, interferons, corticosteroids, cyclophosphamide, azathioprine, methotrexate, and sulfasalazine); antibacterial and antiviral agents; and agents for Alzheimer's treatment (e.g., donepezil, galantamine, memantine and rivastigmine).

Screening Assays

The invention provides methods (also referred to herein as a “screening assays”) for identifying modulators, i.e., candidate or test compounds or agents (e.g., peptides, cyclic peptides, peptidomimetics, small molecules, small organic molecules, or other drugs) which bind to one or more Uba6 enzymes, or have a stimulatory or inhibitory effect on Uba6 expression or on one or more Uba6 activities.

As used herein, the term “small organic molecule” refers to an organic molecule, either naturally occurring or synthetic, that has a molecular weight of more than about 25 daltons and less than about 3000 daltons, preferably less than about 2500 daltons, more preferably less than about 2000 daltons, preferably between about 100 to about 1000 daltons, more preferably between about 200 to about 500 daltons.

In one embodiment, assays for screening candidate or test compounds which bind to or modulate the activity of Uba6, a Uba6 polypeptide or biologically active portion thereof, E2Z, or an E2Z polypeptide or biologically active portion thereof are provided. The test compounds can be obtained using any of the numerous approaches in combinatorial library methods known in the art, including: biological libraries; spatially addressable parallel solid phase or solution phase libraries; synthetic library methods requiring deconvolution; the “one-bead one-compound” library method; and synthetic library methods using affinity chromatography selection. The biological library approach is limited to peptide libraries, while the other four approaches are applicable to peptide, non-peptide oligomer or small molecule libraries of compounds (Lam, K. S. (1997) Anticancer Drug Des. 12:145).

Examples of methods for the synthesis of molecular libraries can be found in the art, for example in: DeWitt et al. (1993) Proc. Natl. Acad. Sci. USA 90:6909; Erb et al. (1994) Proc. Natl. Acad. Sci. USA 91:11422; Zuckermann et al. (1994) J. Med. Chem. 37:2678; Cho et al. (1993) Science 261:1303; Carrell et al. (1994) Angew. Chem. Int. Ed. Engl. 33:2059; Carell et al. (1994) Angew. Chem. Int. Ed. Engl. 33:2061; and in Gallop et al. (1994) J. Med. Chem. 37:1233.

Libraries of compounds may be presented in solution (e.g., Houghten (1992) Biotechniques 13:412), or on beads (Lam (1991) Nature 354:82), chips (Fodor (1993) Nature 364:555), bacteria (Ladner U.S. Pat. No. 5,223,409), spores (Ladner U.S. Pat. No. 5,223,409), plasmids (Cull et al. (1992) Proc. Natl. Acad. Sci. USA 89:1865) or on phage (Scott and Smith (1990) Science 249:386; Devlin (1990) Science 249:404; Cwirla et al. (1990) Proc. Natl. Acad. Sci. USA 87:6378; Felici (1991) J. Mol. Biol. 222:301; Ladner supra).

Examples of methods for introducing a molecular library of randomized nucleic acids into a population of cells can be found in the art, for example in U.S. Pat. No. 6,365,344. A molecular library of randomized nucleic acids can provide for the direct selection of candidate or test compounds with desired phenotypic effects. The general method can involve, for instance, expressing a molecular library of randomized nucleic acids in a plurality of cells, each of the nucleic acids comprising a different nucleotide sequence, screening for a cell of exhibiting a changed physiology in response to the presence in the cell of a candidate or test compound, and detecting and isolating the cell and/or candidate or test compound.

In one embodiment, the introduced nucleic acids are randomized and expressed in the cells as a library of isolated randomized expression products, which may be nucleic acids, such as mRNA, RNAi reagents, antisense RNA, siRNA, ribozyme components, etc., or peptides (e.g., cyclic peptides). For example, RNAi reagents include, but are not limited to, double-stranded or hairpin sequences corresponding to the coding sequence of Uba6 (e.g., a nucleic acid sequence corresponding to the cysteine 625 region of human Uba6 (SEQ ID NO:5) (e.g., the catalytic cysteine domain); a nucleic acid sequence corresponding to amino acids 947 to 1052 of human Uba6 (SEQ ID NO:5) (e.g., the C-terminal ubiquitin-like (Ub1) domain); an adenylate domain; and/or one or two ThiF domains). The library should provide a sufficiently structurally diverse population of randomized expression products to effect a probabilistically sufficient range of cellular responses to provide one or more cells exhibiting a desired response.

The introduced nucleic acids and resultant expression products are randomized, meaning that each nucleic acid and peptide consists of essentially random nucleotides and amino acids, respectively. The library may be fully random or biased, e.g. in nucleotide/residue frequency generally or per position. In other embodiments, the nucleotides or residues are randomized within a defined class, e.g. of hydrophobic amino acids, of purines, etc.

Functional and structural isolation of the randomized expression products may be accomplished by providing free (i.e., not covalently coupled) expression product, though in some situations, the expression product may be coupled to a functional group or fusion partner, preferably a heterologous (to the host cell) or synthetic (not native to any cell) functional group or fusion partner. Exemplary groups or partners include, but are not limited to, signal sequences capable of constitutively localizing the expression product to a predetermined subcellular locale such as the Golgi, endoplasmic reticulum, nucleoli, nucleus, nuclear membrane, mitochondria, chloroplast, secretory vesicles, lysosome, and the like; binding sequences capable of binding the expression product to a predetermined protein while retaining bioactivity of the expression product; sequences signaling selective degradation, of itself or co-bound proteins; and secretory and membrane-anchoring signals.

It may also be desirable to provide a partner which conformationally restricts the randomized expression product to more specifically define the number of structural conformations available to the cell. For example, such a partner may be a synthetic presentation structure: an artificial polypeptide capable of intracellularly presenting a randomized peptide as a conformation-restricted domain. Generally such presentation structures comprise a first portion joined to the N-terminal end of the randomized peptide, and a second portion joined to the C-terminal end of the peptide. Exemplary presentation structures maximize accessibility to the peptide by presenting it on an exterior loop, for example of coiled-coils (Myszka and Chaiken (1994) Biochemistry 33:2362). To increase the functional isolation of the randomized expression product, the presentation structures are selected or designed to have minimal biologically active as expressed in the target cell. In addition, the presentation structures may be modified, randomized, and/or matured to alter the presentation orientation of the randomized expression product. For example, determinants at the base of the loop may be modified to slightly modify the internal loop peptide tertiary structure, while maintaining the absolute amino acid identity. Other presentation structures include zinc-finger domains, loops on beta-sheet turns and coiled-coil stem structures in which non-critical residues are randomized; loop structures held together by cysteine bridges, cyclic peptides, etc.

In one embodiment, an assay is a cell-based assay in which a cell which expresses a Uba6 protein or biologically active portion thereof is contacted with a test compound and the ability of the test compound to modulate one or more Uba6 activities, e.g., ubiquitin activation; ubiquitin-adenylate intermediate formation; ubiquitin thiol esterification; ubiquitin transfer to one or more ubiquitin-conjugating enzymes (e.g., E2s); and/or ubiquitination of one or more target polypeptides is determined. In another embodiment, an assay is a cell-based assay in which a cell which expresses a E2Z protein or biologically active portion thereof is contacted with a test compound and the ability of the test compound to modulate one or more E2Z activities, e.g., ubiquitin activation; ubiquitin-adenylate intermediate formation; ubiquitin thiol esterification; ubiquitin transfer to one or more ubiquitin-protein ligases (e.g., E3s); and/or ubiquitination of one or more target polypeptides is determined.

Determining the ability of the test compound to modulate one or more Uba6 activities and/or one or more E2Z activities can be accomplished by monitoring, for example, ubiquitin activation, formation of a ubiquitin-adenylate intermediate, ubiquitin thiol esterification, ubiquitin transfer to one or more ubiquitin-conjugating enzymes, ubiquitin transfer to one or more ubiquitin-protein ligases and/or ubiquitination of one or more target proteins or polypeptides using, for example, one or more of the assays described herein.

Determining the ability of the test compound to modulate one or more Uba6 and/or E2Z activities can be accomplished, for example, by coupling Uba6 or a Uba6 substrate (e.g., ubiquitin, an E2 and/or a target polypeptide or protein) and/or E2Z or an E2Z substrate (e.g., ubiquitin, an E3 and/or a target polypeptide or protein) with a radioisotope or enzymatic label such that alteration of Uba6, Uba6 substrate, E2Z and/or a E2Z substrate (e.g., by ubiquitination, thiol esterification, ubiquitin adenylation or the like) can be determined by detecting an alteration of the Uba6, Uba6 substrate, E2Z and/or a E2Z substrate (e.g., altered mobility on an SDS-PAGE gel). For example, compounds can be labeled with ¹²⁵I, ³⁵S, ¹⁴C, or ³H, either directly or indirectly, and the radioisotope detected by direct counting of radioemmission or by scintillation counting. Alternatively, compounds can be enzymatically labeled with, for example, horseradish peroxidase, alkaline phosphatase, or luciferase, and the enzymatic label detected by determination of conversion of an appropriate substrate to product.

It is also within the scope of this invention to determine the ability of a compound to interact with Uba6 and/or E2Z without the labeling of any of the interactants. For example, a microphysiometer can be used to detect the interaction of a compound with Uba6 and/or E2Z without the labeling of either the compound or Uba6 and/or E2Z (McConnell, H. M. et al. (1992) Science 257:1906). As used herein, a “microphysiometer” (e.g., Cytosensor) is an analytical instrument that measures the rate at which a cell acidifies its environment using a light-addressable potentiometric sensor (LAPS). Changes in this acidification rate can be used as an indicator of the interaction between a compound and Uba6 and/or E2Z.

In another embodiment, an assay is a cell-based assay comprising contacting a cell expressing Uba6 and/or E2Z (e.g., ubiquitin, an E2, an E3 a target polypeptide or protein) with a test compound and determining the ability of the test compound to modulate (e.g., stimulate or inhibit) the activity of Uba6, a Uba6 target molecule, E2Z and/or an E2Z target molecule. Determining the ability of the test compound to modulate the activity of Uba6, a Uba6 target molecule, E2Z and/or an E2Z target molecule can be accomplished, for example, by determining the ability of the Uba6 and/or the E2Z to bind to modulate ubiquitin activation.

Determining the ability of Uba6 and/or E2Z to modulate ubiquitin activation can be accomplished by one of the methods described above for determining direct binding. In an exemplary embodiment, determining the ability of Uba6 and/or E2Z to modulate ubiquitin activation can be accomplished by detecting one or more Uba6 and/or E2Z activities, e.g., ubiquitin activation; ubiquitin-adenylate intermediate formation; ubiquitin thiol esterification; ubiquitin transfer to one or more ubiquitin-conjugating enzymes (e.g., E2s); ubiquitin transfer to one or more ubiquitin-protein ligases (e.g., E3s); and/or ubiquitination of one or more target polypeptides (e.g., ubiquitin, an E2, an E3, a target polypeptide and/or protein or the like).

In yet another embodiment, an assay of the present invention is a cell-free assay in which Uba6 and/or E2Z is contacted with a test compound and the ability of the test compound to bind to Uba6 and/or E2Z or a biologically active portion of Uba6 and/or E2Z is determined. Biologically active portions of Uba6 to be used in assays of the present invention include, but are not limited to, ThiF domains (e.g., amino acids 50-200 and 460-600 of SEQ ID NO:5, for ThiF domain 1 and ThiF domain 2, respectively), catalytic cysteine domains (e.g., amino acid sequences including cysteine 625 of SEQ ID NO:5), adenylate domains (e.g., amino acids 1-610 of SEQ ID NO:5), C-terminal ubiquitin-like (Ub1) domains (e.g., amino acids 947-1052 of SEQ ID NO:5), and the like. Binding of the test compound to Uba6 and/or E2Z can be determined either directly or indirectly as described above. In an exemplary embodiment, the assay includes contacting Uba6 and/or E2Z or biologically active portion of Uba6 and/or E2Z with a known compound which binds Uba6 and/or E2Z to form an assay mixture, contacting the assay mixture with a test compound, and determining the ability of the test compound to interact with Uba6 and/or E2Z, wherein determining the ability of the test compound to interact with Uba6 and/or E2Z comprises determining the ability of the test compound to preferentially bind to Uba6 and/or E2Z or a biologically active portion of Uba6 and/or E2Z as compared to the known compound.

In another embodiment, the assay is a cell-free assay in which Uba6 and/or E2Z or a biologically active portion of Uba6 and/or E2Z is contacted with a test compound and the ability of the test compound to modulate (e.g., stimulate or inhibit) the activity of Uba6 and/or E2Z or a biologically active portion of Uba6 and/or E2Z is determined. Determining the ability of the test compound to modulate the activity of Uba6 and/or E2Z can be accomplished, for example, by determining the ability of Uba6 and/or E2Z to bind to a Uba6 and/or an E2Z target molecule by one of the methods described above for determining direct binding. Determining the ability of Uba6 and/or E2Z to bind to a Uba6 and/or E2Z target molecule, respectively, can also be accomplished using a technology such as real-time Biomolecular Interaction Analysis (BIA) (Sjolander, S, and Urbaniczky, C. (1991) Anal. Chem. 63:2338-2345 and Szabo et al. (1995) Curr. Opin. Struct. Biol. 5:699-705). As used herein, “BIA” is a technology for studying biospecific interactions in real time, without labeling any of the interactants (e.g., BIAcore). Changes in the optical phenomenon of surface plasmon resonance (SPR) can be used as an indication of real-time reactions between biological molecules.

In yet another embodiment, the cell-free assay involves contacting Uba6 and/or E2Z or a biologically active portion of Uba6 and/or E2Z with a known compound which binds Uba6 and/or E2Z, respectively, to form an assay mixture, contacting the assay mixture with a test compound, and determining the ability of the test compound to interact with Uba6 and/or E2Z, wherein determining the ability of the test compound to interact with Uba6 and/or E2Z comprises determining the ability of Uba6 and/or E2Z to preferentially bind to or modulate the activity of a Uba6 and/or E2Z target molecule (e.g., ubiquitin activation; ubiquitin-adenylate intermediate formation; ubiquitin thiol esterification; ubiquitin transfer to one or more ubiquitin-conjugating enzymes (e.g., E2s); ubiquitin transfer to one or more ubiquitin-protein ligases (e.g., E3s); and/or ubiquitination of one or more target polypeptides (e.g., ubiquitin, an E2, an E3, a target polypeptide and/or protein or the like) and the like).

In more than one embodiment of the above assay methods of the present invention, it may be desirable to immobilize either Uba6, a Uba6 target molecule, E2Z and/or an E2Z target molecule to facilitate separation of complexed from uncomplexed forms of one or both of the proteins, as well as to accommodate automation of the assay. Binding of a test compound to Uba6 and/or E2Z, or interaction of Uba6 and/or E2Z with one or more target molecules in the presence and absence of a candidate compound, can be accomplished in any vessel suitable for containing the reactants. Examples of such vessels include microtitre plates, test tubes, and microfuge tubes. In one embodiment, a fusion protein can be provided which adds a domain that allows one or both of the proteins to be bound to a matrix. For example, glutathione-S-transferase/Uba6 fusion proteins or glutathione-S-transferase/target fusion proteins can be adsorbed onto glutathione sepharose beads (Sigma, St. Louis, Mo.) or glulathione-derivatized microtitre plates, which are then combined with the test compound or the test compound and either the non-adsorbed target protein or Uba6, and the mixture incubated under conditions conducive to complex formation (e.g., at physiological conditions for salt and pH). Following incubation, the beads or microtitre plate wells are washed to remove any unbound components, the matrix immobilized in the case of beads, complex determined either directly or indirectly, for example, as described above. Alternatively, the complexes can be dissociated from the matrix, and the level of Uba6 binding or activity determined using standard techniques.

Other techniques for immobilizing proteins on matrices can also be used in the screening assays of the invention. For example, Uba6, a Uba6 target molecule, E2Z and/or an E2Z target molecule can be immobilized utilizing conjugation of biotin and avidin or streptavidin. Biotinylated Uba6 and/or E2Z or target molecules can be prepared from biotin-NHS (N-hydroxy-succinimide) using techniques known in the art (e.g., biotinylation kit, Pierce Chemicals, Rockford, Ill.), and immobilized in the wells of streptavidin-coated 96 well plates (Pierce). Alternatively, antibodies reactive with Uba6 and/or E2Z or target molecules that do not interfere with binding of Uba6 and/or E2Z to its target molecule can be derivatized to the wells of the plate, and unbound target or Uba6 and/or E2Z trapped in the wells by antibody conjugation. Methods for detecting such complexes, in addition to those described above for the GST-immobilized complexes, include immunodetection of complexes using antibodies reactive with Uba6 and/or E2Z or one or more Uba6 and/or E2Z target molecules, as well as enzyme-linked assays which rely on detecting an enzymatic activity associated with Uba6 and/or E2Z.

In another embodiment, modulators of Uba6 and/or E2Z expression are identified in a method wherein a cell is contacted with a candidate compound and the expression of Uba6 protein, Uba6 mRNA, E2Z protein, and/or E2Z mRNA in the cell is determined. The level of Uba6 protein, Uba6 mRNA, E2Z protein, and/or E2Z mRNA in the presence of the candidate compound is compared to the level of Uba6 protein, Uba6 mRNA, E2Z protein, and/or E2Z mRNA in the absence of the candidate compound. The candidate compound can then be identified as a modulator of Uba6 and/or E2Z protein expression and/or Uba6 and/or E2Z mRNA expression based on this comparison. For example, when expression of Uba6 and/or E2Z protein and/or Uba6 and/or E2Z mRNA is greater (statistically significantly greater) in the presence of the candidate compound than in its absence, the candidate compound is identified as a stimulator of Uba6 and/or E2Z protein expression and/or Uba6 and/or E2Z mRNA expression, respectively. Alternatively, when expression of Uba6 and/or E2Z protein and/or Uba6 and/or E2Z mRNA is less (statistically significantly less) in the presence of the candidate compound than in its absence, the candidate compound is identified as an inhibitor of Uba6 and/or E2Z protein expression and/or Uba6 and/or E2Z mRNA expression, respectively. The level of Uba6 and/or E2Z mRNA or protein expression in the cells can be determined by methods described herein for detecting Uba6 and/or E2Z mRNA or protein.

In yet another aspect of the invention, Uba6 and/or E2Z can be used as “bait proteins” in a two-hybrid assay or three-hybrid assay (see, e.g., U.S. Pat. No. 5,283,317; Zervos et al. (1993) Cell 72:223-232; Madura et al. (1993) J. Biol. Chem. 268:12046-12054; Bartel et al. (1993) Biotechniques 14:920-924; Twabuchi et al. (1993) Oncogene 8:1693-1696; and Brent WO 94/10300), to identify other proteins, which bind to or interact with Uba6 (“Uba6-binding proteins”) and are involved in one or more Uba6 activities (e.g., ubiquitin activation; ubiquitin-adenylate intermediate formation; ubiquitin thiol esterification; ubiquitin transfer to one or more ubiquitin-conjugating enzymes (e.g., E2s); and/or ubiquitination of one or more target polypeptides (e.g., ubiquitin, an E2, a target polypeptide and/or protein or the like)). Alternatively, such Uba6-binding proteins are likely to be Uba6 inhibitors.

In another embodiment, an assay is an animal model based assay comprising contacting a an animal with a test compound and determining the ability of the test compound to alter Uba6 and/or E2Z expression and/or Uba6 and/or E2Z activity. Animals include, but are not limited to, mammals such as non-human primates, rabbits, rats, mice, and the like.

This invention further pertains to novel agents identified by the above-described screening assays. Accordingly, it is within the scope of this invention to further use an agent identified as described herein in an appropriate animal model as described herein. For example, an agent identified as described herein (e.g., a Uba6 and/or E2Z modulating agent) can be used in an animal model to determine the efficacy, toxicity, or side effects of treatment with such an agent. Alternatively, an agent identified as described herein can be used in an animal model to determine the mechanism of action of such an agent. Furthermore, this invention pertains to uses of novel agents identified by the above-described screening assays for treatments of ubiquitin-related disorders (e.g., cellular proliferative disorders and/or neurodegenerative disorders) described herein.

Uba6 Nucleic Acid and Amino Acid Sequences

One aspect of the invention pertains to isolated nucleic acid molecules that encode Uba6 and/or E2Z proteins or biologically active portions thereof, as well as nucleic acid fragments sufficient for use as hybridization probes to identify Uba6- and/or E2Z-encoding nucleic acid molecules (e.g., Uba6 and/or E2Z mRNA, respectively) and fragments for use as PCR primers for the amplification or mutation of Uba6 nucleic acid molecules. As used herein, the term “nucleic acid molecule” is intended to include, but is not limited to, DNA molecules (e.g., cDNA or genomic DNA) and RNA molecules (e.g., mRNA) and analogs of the DNA or RNA generated using nucleotide analogs. The nucleic acid molecule can be single-stranded or double-stranded DNA.

An “isolated” nucleic acid molecule is one which is separated from other nucleic acid molecules which are present in the natural source of the nucleic acid. In an exemplary embodiment, an “isolated” nucleic acid is free of sequences which naturally flank the nucleic acid (i.e., sequences located at the 5′ and 3′ ends of the nucleic acid) in the genomic DNA of the organism from which the nucleic acid is derived. For example, in various embodiments, the isolated Uba6 nucleic acid molecule can contain less than about 5 kb, 4 kb, 3 kb, 2 kb, 1 kb, 0.5 kb or 0.1 kb of nucleotide sequences which naturally flank the nucleic acid molecule in genomic DNA of the cell from which the nucleic acid is derived. Moreover, an “isolated” nucleic acid molecule, such as a cDNA molecule, can be substantially free of other cellular material, or culture medium when produced by recombinant techniques, or substantially free of chemical precursors or other chemicals when chemically synthesized.

A nucleic acid molecule of the present invention, e.g., a nucleic acid molecule having the nucleotide sequence of SEQ ID NO:4 or SEQ ID NO:6, or a portion thereof, can be isolated using standard molecular biology techniques and the sequence information provided herein. Using all or portion of the nucleic acid sequence of SEQ ID NO:4 or SEQ ID NO:6 as a hybridization probe, Uba6 nucleic acid molecules can be isolated using standard hybridization and cloning techniques (e.g., as described in Sambrook, J., Fritsh, E. F., and Maniatis, T., Molecular Cloning: A Laboratory Manual, 2nd, ed., Cold Spring Harbor Laboratory, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989). Moreover, a nucleic acid molecule encompassing all or a portion of SEQ ID NO:4 or SEQ ID NO:6 can be isolated by the polymerase chain reaction (PCR) using synthetic oligonucleotide primers designed based upon the sequence of SEQ ID NO:4 or SEQ ID NO:6.

A nucleic acid of the invention can be amplified using cDNA, mRNA or alternatively, genomic DNA, as a template and appropriate oligonucleotide primers according to standard PCR amplification techniques. The nucleic acid so amplified can be cloned into an appropriate vector and characterized by DNA sequence analysis. Furthermore, oligonucleotides corresponding to Uba6 and/or E2Z nucleotide sequences can be prepared by standard synthetic techniques, e.g., using an automated DNA synthesizer.

In one embodiment, an isolated nucleic acid molecule of the invention comprises the nucleotide sequence shown in SEQ ID NO:4 or SEQ ID NO:6, or a portion of either of these nucleotide sequences. In another embodiment, an isolated nucleic acid molecule of the invention comprises a nucleic acid molecule which is a complement of the nucleotide sequence shown in SEQ ID NO:4 or SEQ ID NO:6, or a portion of any of these nucleotide sequences. A nucleic acid molecule which is complementary to the nucleotide sequence shown in SEQ ID NO:4 or SEQ ID NO:6, is one which is sufficiently complementary to the nucleotide sequence shown in SEQ ID NO:4 or SEQ ID NO:6, such that it can hybridize to the nucleotide sequence shown in SEQ ID NO:4 or SEQ ID NO:6, thereby forming a stable duplex.

In still another embodiment, an isolated nucleic acid molecule of the present invention comprises a nucleotide sequence which is at least about 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or more identical to the entire length of the nucleotide sequence shown in SEQ ID NO:4 or SEQ ID NO:6, or a portion of either of these nucleotide sequences.

Moreover, the nucleic acid molecule of the invention can comprise only a portion of the nucleic acid sequence of SEQ ID NO:4 or SEQ ID NO:6, for example a fragment which can be used as a probe or primer or a fragment encoding a biologically active portion of a Uba6 protein. The nucleic acid sequence of SEQ ID NO:4 or SEQ ID NO:6 allows for the generation of probes and primers designed for use in identifying and/or cloning other Uba6 family members, as well as Uba6 homologues from other species.

The probe/primer typically comprises substantially purified oligonucleotide. The oligonucleotide typically comprises a region of nucleotide sequence that hybridizes under stringent conditions to at least about 12 or 15, about 20 or 25, about 30, 35, 40, 45, 50, 55, 60, 65, or 75 consecutive nucleotides of a sense sequence of SEQ ID NO:4 or SEQ ID NO:6, of an anti-sense sequence of SEQ ID NO:4 or SEQ ID NO:6, or of a naturally occurring allelic variant or mutant of SEQ ID NO:4 or SEQ ID NO:6. In an exemplary embodiment, a nucleic acid molecule of the present invention comprises a nucleotide sequence which is 350-400, 400-450, 450-500, 500-550, 550-600, 600-650, 650-700, 700-750, 750-800, 800-850, 850-900, 900-950, 950-1000, 1000-1500, 1500-2000, 2000-2500, 2500-3000, 3000-3500, 3500-4000, 4000-4500, 4500-5000, 5101, 5386, 5000-5500 or more nucleotides in length and hybridizes under stringent hybridization conditions to a nucleic acid molecule of SEQ ID NO:4 or SEQ ID NO:6.

Probes based on the Uba6 nucleotide sequences can be used to detect transcripts or genomic sequences encoding the same or homologous proteins. In certain embodiments, the probe further comprises a label group attached thereto, e.g., the label group can be a radioisotope, a fluorescent compound, an enzyme, or an enzyme co-factor. Such probes can be used as a part of a diagnostic test kit for identifying cells or tissue which mis-express Uba6, such as by measuring a level of a Uba6-encoding nucleic acid in a sample of cells from a subject e.g., detecting Uba6 mRNA levels or determining whether a genomic Uba6 gene has been mutated or deleted.

A nucleic acid fragment encoding a “biologically active portion of a Uba6 protein” can be prepared by isolating a portion of the nucleotide sequence of SEQ ID NO:4 or SEQ ID NO:6 which encodes a polypeptide having a Uba6 biological activity (the biological activities of the Uba6 proteins are described herein), expressing the encoded portion of the Uba6 protein (e.g., by recombinant expression in vitro) and assessing the activity of the encoded portion of Uba6.

The invention further encompasses nucleic acid molecules that differ from the nucleotide sequence shown in SEQ ID NO:4 or SEQ ID NO:6 due to degeneracy of the genetic code and thus encode the same Uba6 proteins as those encoded by the nucleotide sequence shown in SEQ ID NO:4 or SEQ ID NO:6. In another embodiment, an isolated nucleic acid molecule of the invention has a nucleotide sequence encoding a protein having an amino acid sequence shown in SEQ ID NO:5 or SEQ ID NO:7.

In addition to the Uba6 nucleotide sequences of SEQ ID NO: 4 or SEQ ID NO:6, it will be appreciated by those skilled in the art that DNA sequence polymorphisms that lead to changes in the amino acid sequences of the Uba6 proteins may exist within a population (e.g., the human population). Such genetic polymorphism in the Uba6 genes may exist among individuals within a population due to natural allelic variation. As used herein, the terms “gene” and “recombinant gene” refer to nucleic acid molecules which include an open reading frame encoding Uba6, such as a mammalian or mouse or zebrafish Uba6, and can further include non-coding regulatory sequences, and introns.

Allelic variants of Uba6 include both functional and non-functional Uba6 proteins. Functional allelic variants are naturally occurring amino acid sequence variants of the Uba6 that maintain the ability to bind a Uba6 ligand and/or modulate any of the Uba6 activities described herein. Functional allelic variants will typically contain only conservative substitution of one or more amino acids of SEQ ID NO:5 or SEQ ID NO:7 or substitution, deletion or insertion of non-critical residues in non-critical regions of the protein.

Non-functional allelic variants are naturally occurring amino acid sequence variants of the Uba6 that do not have the ability to either bind a Uba6 ligand and/or modulate any of the Uba6 activities described herein. Non-functional allelic variants will typically contain a non-conservative substitution, a deletion, or insertion or premature truncation of the amino acid sequence of SEQ ID NO:5 or SEQ ID NO:7 or a substitution, insertion or deletion in critical residues or critical regions.

The present invention further provides non-human orthologs of a Uba6 protein. Orthologs of a Uba6 protein are proteins that are isolated from various organisms that possess the same Uba6 ligand binding and/or modulation of any of the Uba6 activities described herein. Orthologs of Uba6 can readily be identified as comprising an amino acid sequence that is substantially identical to SEQ ID NO:5 or SEQ ID NO:7.

Moreover, nucleic acid molecules encoding other Uba6 family members and, thus, which have a nucleotide sequence which differs from the Uba6 sequences of SEQ ID NO:4 or SEQ ID NO:6 are intended to be within the scope of the invention. For example, another Uba6 cDNA can be identified based on the nucleotide sequence of human or mouse Uba6. Moreover, nucleic acid molecules encoding Uba6 proteins from different species, and thus which have a nucleotide sequence which differs from the Uba6 sequences of SEQ ID NO:4 or SEQ ID NO:6 are intended to be within the scope of the invention. For example, a monkey Uba6 cDNA can be identified based on the nucleotide sequence of a human or mouse Uba6.

Nucleic acid molecules corresponding to natural allelic variants and homologues of the Uba6 cDNAs of the invention can be isolated based on their homology to the Uba6 nucleic acids disclosed herein using the cDNAs disclosed herein, or a portion thereof, as a hybridization probe according to standard hybridization techniques under stringent hybridization conditions.

Accordingly, in another embodiment, an isolated nucleic acid molecule of the invention is at least 15, 20, 25, 30 or more nucleotides in length and hybridizes under stringent conditions to the nucleic acid molecule comprising the nucleotide sequence of SEQ ID NO:4 or SEQ ID NO:6. In other embodiment, the nucleic acid is at least 30, 50, 100, 150, 200, 250, 300, 307, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000, 1500, 2000, 2500, 3000, 3500, 4000, 4500, 5000, 5101, 5386, or 5500 nucleotides in length. As used herein, the term “hybridizes under stringent conditions” is intended to describe conditions for hybridization and washing under which nucleotide sequences at least 60% identical to each other typically remain hybridized to each other. In exemplary embodiments, the conditions are such that sequences at least about 70%, at least about 80%, at least about 85%, at least about 90% or 95% identical to each other typically remain hybridized to each other. Such stringent conditions are known to those skilled in the art and can be found in Current Protocols in Molecular Biology, John Wiley & Sons, N.Y. (1989), 6.3.1-6.3.6. A non-limiting example of stringent hybridization conditions are hybridization in 6× sodium chloride/sodium citrate (SSC) at about 45° C., followed by one or more washes in 0.2×SSC, 0.1% SDS at 50° C., in certain aspects at 55° C., and in other aspects at 60° C. or 65° C. In certain aspects, an isolated nucleic acid molecule of the invention that hybridizes under stringent conditions to the sequence of SEQ ID NO:4 or SEQ ID NO:6 corresponds to a naturally-occurring nucleic acid molecule. As used herein, a “naturally-occurring” nucleic acid molecule refers to an RNA or DNA molecule having a nucleotide sequence that occurs in nature (e.g., encodes a natural protein).

In addition to naturally-occurring allelic variants of the Uba6 sequences that may exist in the population, the skilled artisan will further appreciate that changes can be introduced by mutation into the nucleotide sequences of SEQ ID NO:4 or SEQ ID NO:6, thereby leading to changes in the amino acid sequence of the encoded Uba6 proteins, without altering the functional ability of the Uba6 proteins. For example, nucleotide substitutions leading to amino acid substitutions at “non-essential” amino acid residues can be made in the sequence of SEQ ID NO:4 or SEQ ID NO:6. A “non-essential” amino acid residue is a residue that can be altered from the wild-type sequence of Uba6 (e.g., the sequence of SEQ ID NO:5 or SEQ ID NO:7) without altering the biological activity, whereas an “essential” amino acid residue is required for biological activity. For example, amino acid residues that are conserved among the Uba6 proteins of the present invention, are predicted to be particularly unamenable to alteration. Furthermore, additional amino acid residues that are conserved between the Uba6 proteins of the present invention and other members of the Uba6 family of proteins are not likely to be amenable to alteration.

Accordingly, another aspect of the invention pertains to nucleic acid molecules encoding Uba6 proteins that contain changes in amino acid residues that are not essential for activity. Such Uba6 proteins differ in amino acid sequence from SEQ ID NO:5 or SEQ ID NO:7, yet retain biological activity. In one embodiment, the isolated nucleic acid molecule comprises a nucleotide sequence encoding a protein, wherein the protein comprises an amino acid sequence at least about 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or more identical to SEQ ID NO:5 or SEQ ID NO:7.

An isolated nucleic acid molecule encoding a Uba6 protein homologous to the protein of SEQ ID NO:5 or SEQ ID NO:7 can be created by introducing one or more nucleotide substitutions, additions or deletions into the nucleotide sequence of SEQ ID NO:4 or SEQ ID NO:6, such that one or more amino acid substitutions, additions or deletions are introduced into the encoded protein. Mutations can be introduced into SEQ ID NO:4 or SEQ ID NO:6 by standard techniques, such as site-directed mutagenesis and PCR-mediated mutagenesis. In certain embodiments, conservative amino acid substitutions are made at one or more predicted non-essential amino acid residues. A “conservative amino acid substitution” is one in which the amino acid residue is replaced with an amino acid residue having a similar side chain. Families of amino acid residues having similar side chains have been defined in the art. These families include amino acids with basic side chains (e.g., lysine, arginine, histidine), acidic side chains (e.g., aspartic acid, glutamic acid), uncharged polar side chains (e.g., glycine, asparagine, glutamine, serine, threonine, tyrosine, cysteine), nonpolar side chains (e.g., alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine, tryptophan), beta-branched side chains (e.g., threonine, valine, isoleucine) and aromatic side chains (e.g., tyrosine, phenylalanine, tryptophan, histidine). In certain aspects, a predicted nonessential amino acid residue in a Uba6 protein is replaced with another amino acid residue from the same side chain family. Alternatively, in another embodiment, mutations can be introduced randomly along all or part of a Uba6 coding sequence, such as by saturation mutagenesis, and the resultant mutants can be screened for Uba6 biological activity to identify mutants that retain activity. Following mutagenesis of SEQ ID NO:4 or SEQ ID NO:6, the encoded protein can be expressed recombinantly and the activity of the protein can be determined.

In an exemplary embodiment, a mutant Uba6 protein can be assayed for the ability to (1) activate ubiquitin; (2) mediate ubiquitin-adenylate intermediate formation; (3) mediate ubiquitin thiol esterification; (4) transfer ubiquitin to one or more ubiquitin-conjugating enzymes (e.g., E2s); and/or (5) ubiquitinate one or more target polypeptides (e.g., ubiquitin, an E2, a target polypeptide and/or protein or the like). In another exemplary embodiment, a mutant E2Z protein can be assayed for the ability to (1) activate ubiquitin; (2) mediate ubiquitin-adenylate intermediate formation; (3) mediate ubiquitin thiol esterification; (4) transfer ubiquitin to one or more ubiquitin-protein ligases (e.g., E3s); and/or (5) ubiquitinate one or more target polypeptides (e.g., ubiquitin, an E3, a target polypeptide and/or protein or the like).

In addition to the nucleic acid molecules encoding Uba6 and/or E2Z proteins described above, another aspect of the invention pertains to isolated nucleic acid molecules which are antisense thereto. An “antisense” nucleic acid comprises a nucleotide sequence which is complementary to a “sense” nucleic acid encoding a protein, e.g., complementary to the coding strand of a double-stranded cDNA molecule or complementary to an mRNA sequence. Accordingly, an antisense nucleic acid can hydrogen bond to a sense nucleic acid. The antisense nucleic acid can be complementary to an entire Uba6 and/or E2Z coding strand, or to only a portion thereof. In one embodiment, an antisense nucleic acid molecule is antisense to a “coding region” of the coding strand of a nucleotide sequence encoding Uba6 and/or E2Z. The term “coding region” refers to the region of the nucleotide sequence comprising codons which are translated into amino acid residues. In another embodiment, the antisense nucleic acid molecule is antisense to a “non-coding region” of the coding strand of a nucleotide sequence encoding Uba6 and/or E2Z. The term “non-coding region” refers to 5′ and 3′ sequences which flank the coding region that are not translated into amino acids (i.e., also referred to as 5′ and 3′ untranslated regions).

Embodiments of the present invention provide amino acid sequences having one or more biologically active portions of a Uba6 and/or E2Z protein. As used herein, a “biologically active portion of a Uba6 protein” includes a fragment of a Uba6 protein which participates in an interaction between a Uba6 molecule and a non-Uba6 molecule. As used herein, a “biologically active portion of an E2Z protein” includes a fragment of a E2Z protein which participates in an interaction between an E2Z molecule and a non-E2Z molecule. Biologically active portions of a Uba6 protein include peptides comprising amino acid sequences sufficiently identical to or derived from the amino acid sequence of the Uba6 protein, e.g., the amino acid sequence shown in SEQ ID NO:5 or SEQ ID NO:7, which include less amino acids than a full length Uba6 protein, and exhibit at least one activity of a Uba6 protein. Typically, biologically active portions comprise a domain or motif (e.g., one, two or more ThiF domains, one or more catalytic cysteine domains, one or more adenylate domains and/or one or more C-terminal ubiquitin-like (Ub1) domains) with at least one activity of the Uba6 protein (e.g., ubiquitin activation; ubiquitin-adenylate intermediate formation; ubiquitin thiol esterification; ubiquitin transfer to one or more ubiquitin-conjugating enzymes (e.g., E2s); and/or ubiquitination of one or more target polypeptides (e.g., ubiquitin, an E2, a target polypeptide and/or protein or the like). A biologically active portion of a Uba6 protein can be a polypeptide which is, for example, 10, 25, 50, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1050, 1052, 1053 or more amino acids in length. Biologically active portions of a Uba6 protein can be used as targets for developing agents which modulate a UPP activity. In one embodiment, a biologically active portion of a Uba6 protein comprises at least one ThiF domain, catalytic cysteine domain, adenylate domain and/or a C-terminal ubiquitin-like domain.

It is to be understood that in certain embodiments, a biologically active portion of a Uba6 protein of the present invention may contain at least one of the above-identified structural domains. In other embodiments, a biologically active portion of a Uba6 protein may contain at least two, at least three or at least four of the above-identified structural domains. Moreover, other biologically active portions, in which other regions of the protein are deleted, can be prepared by recombinant techniques and evaluated for one or more of the functional activities of a native Uba6 protein.

In an exemplary embodiment, the Uba6 protein has an amino acid sequence shown in SEQ ID NO:5 or SEQ ID NO:7. In other embodiments, the Uba6 protein is substantially homologous to SEQ ID NO:5 or SEQ ID NO:7, and retains the functional activity of the protein of SEQ ID NO:5 or SEQ ID NO:7, yet differs in amino acid sequence due to natural allelic variation or mutagenesis, as described in detail above. Accordingly, in another embodiment, the Uba6 protein is a protein which comprises an amino acid sequence at least about 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or more identical to SEQ ID NO:5 or SEQ ID NO:7.

To determine the percent identity of two amino acid sequences or of two nucleic acid sequences, the sequences are aligned for optimal comparison purposes (e.g., gaps can be introduced in one or both of a first and a second amino acid or nucleic acid sequence for optimal alignment and non-homologous sequences can be disregarded for comparison purposes). In an exemplary embodiment, the length of a reference sequence aligned for comparison purposes is at least 30%, at least 40%, at least 50%, at least 60%, and even at least 70%, 80%, or 90% of the length of the reference sequence (e.g., when aligning a second sequence to the Uba6 amino acid sequence having 1052 and 1053 amino acid residues, respectively, at least 315, at least 420, at least 526, at least 631, and even at least 736, 841 or 946 amino acid residues are aligned). The amino acid residues or nucleotides at corresponding amino acid positions or nucleotide positions are then compared. When a position in the first sequence is occupied by the same amino acid residue or nucleotide as the corresponding position in the second sequence, then the molecules are identical at that position (as used herein amino acid or nucleic acid “identity” is equivalent to amino acid or nucleic acid “homology”). The percent identity between the two sequences is a function of the number of identical positions shared by the sequences, taking into account the number of gaps, and the length of each gap, which need to be introduced for optimal alignment of the two sequences.

The comparison of sequences and determination of percent identity between two sequences can be accomplished using a mathematical algorithm. In an exemplary embodiment, the percent identity between two amino acid sequences is determined using the Needleman and Wunsch (J. Mol. Biol. (48):444-453 (1970)) algorithm which has been incorporated into the GAP program in the GCG software package (available at http://www.gcg.com), using either a Blosum 62 matrix or a PAM250 matrix, and a gap weight of 16, 14, 12, 10, 8, 6, or 4 and a length weight of 1, 2, 3, 4, 5, or 6. In yet another exemplary embodiment, the percent identity between two nucleotide sequences is determined using the GAP program in the GCG software package (available at the website gcg.com), using a NWSgapdnaCMP matrix and a gap weight of 40, 50, 60, 70, or 80 and a length weight of 1, 2, 3, 4, 5, or 6. In another embodiment, the percent identity between two amino acid or nucleotide sequences is determined using the algorithm of E. Meyers and W. Miller (CABIOS (1989) 4:11) which has been incorporated into the ALIGN program (version 2.0), using a PAM120 weight residue table, a gap length penalty of 12 and a gap penalty of 4.

The nucleic acid and protein sequences of the present invention can further be used as a “query sequence” to perform a search against public databases to, for example, identify other family members or related sequences. Such searches can be performed using the NBLAST and XBLAST programs (version 2.0) of Altschul, et al. (1990) J. Mol. Biol. 215:403. BLAST nucleotide searches can be performed with the NBLAST program, score=100, wordlength=12 to obtain nucleotide sequences homologous to Uba6 nucleic acid molecules of the invention. BLAST protein searches can be performed with the XBLAST program, score=50, wordlength=3 to obtain amino acid sequences homologous to Uba6 protein molecules of the invention. To obtain gapped alignments for comparison purposes, Gapped BLAST can be utilized as described in Altschul et al., (1997) Nucleic Acids Res. 25(17):3389. When utilizing BLAST and Gapped BLAST programs, the default parameters of the respective programs (e.g., XBLAST and NBLAST) can be used (ncbi.nlm.nih.gov website).

Recombinant Expression Vectors and Host Cells

Another aspect of the invention pertains to vectors, such as expression vectors, containing a nucleic acid encoding a Uba6 and/or an E2Z protein (or a portion thereof). As used herein, the term “vector” refers to a nucleic acid molecule capable of transporting another nucleic acid to which it has been linked. One type of vector is a “plasmid,” which refers to a circular double stranded DNA loop into which additional DNA segments can be ligated. Another type of vector is a viral vector, wherein additional DNA segments can be ligated into the viral genome. Certain vectors are capable of autonomous replication in a host cell into which they are introduced (e.g., bacterial vectors having a bacterial origin of replication and episomal mammalian vectors). Other vectors (e.g., non-episomal mammalian vectors) are integrated into the genome of a host cell upon introduction into the host cell, and thereby are replicated along with the host genome. Moreover, certain vectors are capable of directing the expression of genes to which they are operatively linked. Such vectors are referred to herein as “expression vectors.” In general, expression vectors of utility in recombinant DNA techniques are often in the form of plasmids. In the present specification, “plasmid” and “vector” can be used interchangeably as the plasmid is the most commonly used form of vector. However, the invention is intended to include such other forms of expression vectors, such as viral vectors (e.g., replication defective retroviruses, adenoviruses and adeno-associated viruses), which serve equivalent functions.

The recombinant expression vectors of the invention comprise a nucleic acid of the invention in a form suitable for expression of the nucleic acid in a host cell, which means that the recombinant expression vectors include one or more regulatory sequences, selected on the basis of the host cells to be used for expression, which is operatively linked to the nucleic acid sequence to be expressed. Within a recombinant expression vector, “operably linked” is intended to mean that the nucleotide sequence of interest is linked to the regulatory sequence(s) in a manner which allows for expression of the nucleotide sequence (e.g., in an in vitro transcription/translation system or in a host cell when the vector is introduced into the host cell). The term “regulatory sequence” is intended to includes promoters, enhancers and other expression control elements (e.g., polyadenylation signals). Such regulatory sequences are described, for example, in Goeddel, Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, Calif. (1990). Regulatory sequences include those which direct constitutive expression of a nucleotide sequence in many types of host cells and those which direct expression of the nucleotide sequence only in certain host cells (e.g., tissue-specific regulatory sequences). It will be appreciated by those skilled in the art that the design of the expression vector can depend on such factors as the choice of the host cell to be transformed, the level of expression of protein desired, etc. The expression vectors of the invention can be introduced into host cells to thereby produce proteins or peptides, including fusion proteins or peptides, encoded by nucleic acids as described herein (e.g., Uba6 and/or E2Z proteins, mutant forms of Uba6 and/or E2Z proteins, fusion proteins, and the like).

The recombinant expression vectors of the invention can be designed for expression of Uba6 in prokaryotic or eukaryotic cells. For example, Uba6, Uba6 fragments, E2Z and/or E2Z fragments can be expressed in bacterial cells such as E. coli, insect cells (using baculovirus expression vectors), yeast cells or mammalian cells. Suitable host cells are discussed further in Goeddel, Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, Calif. (1990). Alternatively, the recombinant expression vector can be transcribed and translated in vitro, for example using T7 promoter regulatory sequences and T7 polymerase.

Expression of polypeptides in prokaryotes is most often carried out in E. coli with vectors containing constitutive or inducible promoters directing the expression of either fusion or non-fusion proteins. Fusion vectors add a number of amino acids to a polypeptide encoded therein, usually to the amino terminus of the recombinant polypeptide. Such fusion vectors typically serve three purposes: 1) to increase expression of recombinant polypeptide; 2) to increase the solubility of the recombinant polypeptide; and 3) to aid in the purification of the recombinant polypeptide by acting as a ligand in affinity purification. Often, in fusion expression vectors, a proteolytic cleavage site is introduced at the junction of the fusion moiety and the recombinant polypeptide to enable separation of the recombinant polypeptide from the fusion moiety subsequent to purification of the fusion polypeptide. Such enzymes, and their cognate recognition sequences, include Factor Xa, thrombin and enterokinase. Typical fusion expression vectors include pGEX (Pharmacia Biotech Inc; Smith, D. B. and Johnson, K. S. (1988) Gene 67:31-40), pMAL (New England Biolabs, Beverly, Mass.) and pRIT5 (Pharmacia, Piscataway, N.J.) which fuse glutathione S-transferase (GST), maltose E binding protein, or protein A, respectively, to the target recombinant protein.

Examples of suitable inducible non-fusion E. coli expression vectors include pTrc (Amann et al., (1988) Gene 69:301-315) and pET 11d (Studier et al., Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, Calif. (1990) 60-89). Target gene expression from the pTrc vector relies on host RNA polymerase transcription from a hybrid trp-lac fusion promoter. Target gene expression from the pET 11d vector relies on transcription from a T7 gn10-lac fusion promoter mediated by a coexpressed viral RNA polymerase (T7 gn1). This viral polymerase is supplied by host strains BL21(DE3) or HMS174(DE3) from a resident prophage harboring a T7 gn1 gene under the transcriptional control of the lacUV 5 promoter.

One strategy to maximize recombinant protein expression in E. coli is to express the protein in a host bacteria with an impaired capacity to proteolytically cleave the recombinant protein (Gottesman, S., Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, Calif. (1990) 119-128). Another strategy is to alter the nucleic acid sequence of the nucleic acid to be inserted into an expression vector so that the individual codons for each amino acid are those preferentially utilized in E. coli (Wada et al., (1992) Nucleic Acids Res. 20:2111). Such alteration of nucleic acid sequences of the invention can be carried out by standard DNA synthesis techniques.

In another embodiment, the Uba6 expression vector is a yeast expression vector. Examples of vectors for expression in yeast S. cerevisiae include pYepSec1 (Baldari et al., (1987) Embo J. 6:229), pMFa (Kurjan and Herskowitz, (1982) Cell 30:933), pJRY88 (Schultz et al., (1987) Gene 54:113), pYES2 (Invitrogen, San Diego, Calif.), and picZ (Invitrogen, San Diego, Calif.).

Alternatively, Uba6 and/or E2Z polypeptides can be expressed in insect cells using baculovirus expression vectors. Baculovirus vectors available for expression of proteins in cultured insect cells (e.g., Sf 9 cells) include the pAc series (Smith et al. (1983) Mol. Cell. Biol. 3:2156) and the pVL series (Lucklow and Summers (1989) Virology 170:31).

In yet another embodiment, a nucleic acid of the invention is expressed in mammalian cells using a mammalian expression vector. Examples of mammalian expression vectors include pCDM8 (Seed, B. (1987) Nature 329:840) and pMT2PC (Kaufman et al. (1987) EMBO J. 6:187). When used in mammalian cells, the expression vector's control functions are often provided by viral regulatory elements. For example, commonly used promoters are derived from polyoma, adenovirus 2, cytomegalovirus and Simian virus 40. For other suitable expression systems for both prokaryotic and eukaryotic cells see chapters 16 and 17 of Sambrook, J., Fritsh, E. F., and Maniatis, T., Molecular Cloning: A Laboratory Manual, 2nd, ed., Cold Spring Harbor Laboratory, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989.

In another embodiment, the recombinant mammalian expression vector is capable of directing expression of the nucleic acid preferentially in a particular cell type (e.g., tissue-specific regulatory elements are used to express the nucleic acid). Tissue-specific regulatory elements are known in the art. Non-limiting examples of suitable tissue-specific promoters include the albumin promoter (liver-specific; Pinkert et al. (1987) Genes Dev. 1:268), lymphoid-specific promoters (Calame and Eaton (1988) Adv. Immunol. 43:235), in particular promoters of T cell receptors (Winoto and Baltimore (1989) EMBO J. 8:729) and immunoglobulins (Banerji et al. (1983) Cell 33:729; Queen and Baltimore (1983) Cell 33:741), neuron-specific promoters (e.g., the neurofilament promoter; Byrne and Ruddle (1989) Proc. Natl. Acad. Sci. U.S.A. 86:5473), pancreas-specific promoters (Edlund et al. (1985) Science 230:912), and mammary gland-specific promoters (e.g., milk whey promoter; U.S. Pat. No. 4,873,316 and European Application Publication No. 264,166). Developmentally regulated promoters are also encompassed, for example the murine hox promoters (Kessel and Gruss (1990) Science 249:374) and the α-fetoprotein promoter (Campes and Tilghman (1989) Genes Dev. 3:537).

In one embodiment, the present invention provides a nucleic acid molecule which is antisense to a Uba6 nucleic acid molecule. As used herein, the term “antisense” refers to a nucleic acid that interferes with the function of DNA and/or RNA and may result in suppression of expression of the RNA and/or DNA. The antisense nucleic acid comprises a nucleotide sequence which is complementary to a “sense” nucleic acid encoding a protein, e.g., complementary to the coding strand of a double-stranded cDNA molecule or complementary to an mRNA sequence. Accordingly, an antisense nucleic acid can hydrogen bond to a sense nucleic acid. The antisense nucleic acid can be complementary to an entire Uba6 coding strand, or to only a portion thereof.

An antisense nucleic acid molecule can be delivered to a cell to express an exogenous nucleotide sequence, to inhibit, eliminate, augment, or alter expression of an endogenous nucleotide sequence, or to express a specific physiological characteristic not naturally associated with the cell. In certain embodiments, the antisense nucleic acid is an antisense RNA, an interfering double stranded RNA (“dsRNA”), a short interfering RNA (“siRNA”) or a ribozyme.

As used herein, the term “siRNA” refers to double-stranded RNA that is less than 30 bases, such as 21-25 bases in length. siRNA may be prepared by any method known in the art. For a review of siRNA and RNA interference, see Macrae et al. (2006) Science 311:195; Vermeulen et al. (2005) RNA 11:674; Nishikura (2001) Cell 16:415; Fire et al. (1998) Nature 391:806. In one embodiment, single-stranded, gene-specific sense and antisense RNA oligomers with overhanging 3′ deoxynucleotides are prepared and purified. For example, two oligomers, can be annealed by heating to 94° C. for 2 minutes, cooling to 90° C. for 1 minute, and then cooling to 20° C. at a rate of 1° C. per minute. The siRNA can then be injected into an animal or delivered into a desired cell type using methods of nucleic acid delivery described herein.

Another aspect of the invention pertains to host cells into which a recombinant expression vector of the invention has been introduced, containing sequences which allow it to homologously recombine into a specific site of the host cell's genome. The terms “host cell” and “recombinant host cell” are used interchangeably herein. It is understood that such terms refer not only to the particular subject cell but to the progeny or potential progeny of such a cell. Because certain modifications may occur in succeeding generations due to either mutation or environmental influences, such progeny may not, in fact, be identical to the parent cell, but are still included within the scope of the term as used herein.

A host cell can be any prokaryotic or eukaryotic cell. For example, host cells can be bacterial cells such as E. coli, insect cells (e.g., Drosophila cells), yeast, Xenopus cells, zebrafish cells, or mammalian cells (such as Chinese hamster ovary cells (CHO), African green monkey kidney cells (COS), or fetal human cells (293T)). Other suitable host cells are known to those skilled in the art.

Vector DNA can be introduced into prokaryotic or eukaryotic cells via conventional transformation or transfection techniques. As used herein, the terms “transformation” and “transfection” are intended to refer to a variety of art-recognized techniques for introducing foreign nucleic acid (e.g., DNA) into a host cell, including calcium phosphate or calcium chloride co-precipitation, DEAE-dextran-mediated transfection, lipofection, or electroporation. Suitable methods for transforming or transfecting host cells can be found in Sambrook, et al. (Molecular Cloning: A Laboratory Manual, 2nd, ed., Cold Spring Harbor Laboratory, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989), and other laboratory manuals.

For stable transfection of mammalian cells, it is known that, depending upon the expression vector and transfection technique used, only a small fraction of cells may integrate the foreign DNA into their genome. In order to identify and select these integrants, a gene that encodes a selectable marker (e.g., resistance to antibiotics) is generally introduced into the host cells along with the gene of interest. Exemplary selectable markers include those which confer resistance to drugs, such as G418, hygromycin and methotrexate. Nucleic acid encoding a selectable marker can be introduced into a host cell on the same vector as that encoding a detectable translation product or can be introduced on a separate vector. Cells stably transfected with the introduced nucleic acid can be identified by drug selection (e.g., cells that have incorporated the selectable marker gene will survive, while the other cells die).

A host cell of the invention, such as a prokaryotic or eukaryotic host cell in culture, can be used to produce (i.e., express) a Uba6 and/or an E2Z protein or portion thereof. Accordingly, the invention further provides methods for producing a Uba6 and/or an E2Z protein or portion thereof using the host cells of the invention. In one embodiment, the method comprises culturing the host cell of invention (into which a recombinant expression vector encoding a detectable translation product has been introduced) in a suitable medium such that a detectable translation product is produced. In another embodiment, the method further comprises isolating a Uba6 and/or an E2Z protein or portion thereof from the medium or the host cell.

The host cells of the invention can also be used to produce nonhuman transgenic animals. For example, in one embodiment, a host cell of the invention is a fertilized oocyte or an embryonic stem cell into which Uba6-coding sequences have been introduced. Such host cells can then be used to create non-human transgenic animals in which exogenous Uba6 sequences have been introduced into their genome. As used herein, a “transgenic animal” is a non-human animal, such as a mammal (e.g., a rodent such as a rat or mouse), in which one or more of the cells of the animal includes a transgene. Other examples of transgenic animals include non-human primates, sheep, dogs, cows, goats, chickens, amphibians, etc. A transgene is exogenous DNA which is integrated into the genome of a cell from which a transgenic animal develops and which remains in the genome of the mature animal, thereby directing the expression of an encoded gene product in one or more cell types or tissues of the transgenic animal. As used herein, a “homologous recombinant animal” is a non-human animal such as a mamma (e.g., a mouse), in which an endogenous gene has been altered by homologous recombination between the endogenous gene and an exogenous DNA molecule introduced into a cell of the animal, e.g., an embryonic cell of the animal, prior to development of the animal.

Methods for generating transgenic animals via embryo manipulation and microinjection, particularly animals such as mice, have become conventional in the art and are described, for example, in U.S. Pat. Nos. 4,736,866 and 4,870,009, in U.S. Pat. No. 4,873,191 by Wagner et al., in Hogan, B., Manipulating the Mouse Embryo, (Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1986), and in Wilmut et al. (1997) Nature 385:810. Similar methods are used for production of other transgenic animals. Methods for producing transgenic non-humans animals that contain selected systems which allow for regulated expression of the transgene are described in Lakso et al. (1992) Proc. Natl. Acad. Sci. U.S.A. 89:6232; and O'Gorman et al. (1991) Science 251:1351).

Diagnostic Assays

An exemplary method for detecting Uba6 and/or E2Z expression or Uba6 and/or E2Z activity in a biological sample involves obtaining a biological sample from a test subject and contacting the biological sample with a compound or an agent capable of detecting Uba6 and/or E2Z expression or Uba6 and/or E2Z activity (e.g., ubiquitin activation; ubiquitin-adenylate intermediate formation; ubiquitin thiol esterification; ubiquitin transfer to one or more ubiquitin-conjugating enzymes (e.g., E2s); ubiquitin transfer to one or more ubiquitin-protein ligases (e.g., E3s); and/or ubiquitination of one or more target polypeptides).

In certain aspects, an agent for detecting Uba6 and/or E2Z expression or Uba6 and/or E2Z activity (e.g., ubiquitin activation; ubiquitin-adenylate intermediate formation; ubiquitin thiol esterification; ubiquitin transfer to one or more ubiquitin-conjugating enzymes (e.g., E2s); ubiquitin transfer to one or more ubiquitin-protein ligases (e.g., E3s); and/or ubiquitination of one or more target polypeptides) is an antibody capable of binding to Uba6, ubiquitin and/or a target polypeptide or protein, such as an antibody. Antibodies can be polyclonal or monoclonal. An intact antibody, or a fragment thereof (e.g., Fab or F(ab′)₂) can be used. The term “labeled,” with regard to the probe or antibody, is intended to encompass direct labeling of the probe or antibody by coupling (i.e., physically linking) a detectable substance to the probe or antibody, as well as indirect labeling of the probe or antibody by reactivity with another reagent that is directly labeled. Examples of indirect labeling include detection of a primary antibody using a fluorescently labeled secondary antibody and end-labeling of a DNA probe with biotin such that it can be detected with fluorescently labeled streptavidin.

The term “biological sample” is intended to include tissues, cells and biological fluids isolated from a subject, as well as tissues, cells and fluids present within a subject. That is, the detection method of the invention can be used to detect Uba6 and/or E2Z expression or Uba6 and/or E2Z activity in a biological sample in vitro as well as in vivo. For example, in vitro techniques for detection of Uba6 and/or E2Z include enzyme linked immunosorbent assays (ELISAs), Western blots, immunoprecipitations and immunofluorescence. Furthermore, in vivo techniques for detection of Uba6 and/or E2Z expression or Uba6 and/or E2Z activity include introducing into a subject a labeled anti-Uba6 and/or an anti-E2Z antibody, a labeled anti-ubiquitin antibody, a labeled anti-E2 antibody, a labeled anti-E3 antibody, a labeled anti-target protein or the like. For example, the antibody can be labeled with a radioactive marker whose presence and location in a subject can be detected by standard imaging techniques.

In one embodiment, the biological sample contains protein molecules from the test subject. In certain aspects, a biological sample is a serum sample isolated by conventional means from a subject.

In another embodiment, the methods further involve obtaining a control biological sample from a control subject, contacting the control sample with a compound or agent capable of detecting Uba6 and/or E2Z expression or Uba6 and/or E2Z activity, such that the presence of Uba6 and/or E2Z expression or Uba6 and/or E2Z activity is detected in the biological sample, and comparing the presence of Uba6 and/or E2Z expression or Uba6 activity in the control sample with the presence of Uba6 and/or E2Z expression or Uba6 and/or E2Z activity in the test sample.

The invention also encompasses kits for detecting the presence of Uba6 and/or E2Z expression or Uba6 and/or E2Z activity in a biological sample. For example, the kit can comprise a labeled compound or agent capable of detecting Uba6 and/or E2Z expression or Uba6 and/or E2Z activity in a biological sample; means for determining the amount of Uba6 and/or E2Z expression or Uba6 and/or E2Z activity in the sample; and means for comparing the amount of Uba6 and/or E2Z expression or Uba6 and/or E2Z activity in the sample with a standard. The compound or agent can be packaged in a suitable container. The kit can further comprise instructions for using the kit to detect Uba6 and/or E2Z expression or Uba6 and/or E2Z activity.

Prognostic Assays

The diagnostic methods described herein can furthermore be utilized to identify subjects having or at risk of developing a disease or disorder associated with aberrant Uba6 expression or activity (e.g., cancer). For example, the assays described herein, such as the preceding diagnostic assays or the following assays, can be utilized to identify a subject having or at risk of developing a disorder associated with a misregulation in Uba6 expression and/or activity, such as ubiquitin-related disorders such as cancer, inflammatory disorders, neurodegenerative disorders and the like.

Thus, the present invention provides a method for identifying a disease or disorder associated with aberrant Uba6 and/or E2Z expression or activity in which a test sample is obtained from a subject and Uba6 and/or E2Z expression and/or Uba6 and/or E2Z activity is detected, wherein the presence of Uba6 and/or E2Z expression and/or Uba6 and/or E2Z activity is diagnostic for a subject having or at risk of developing a disease or disorder associated with aberrant Uba6 and/or E2Z expression and/or Uba6 and/or E2Z activity expression or activity. As used herein, a “test sample” refers to a biological sample obtained from a subject of interest. For example, a test sample can be a biological fluid (e.g., serum), cell sample, or tissue.

Furthermore, the prognostic assays described herein can be used to determine whether a subject can be administered an agent (e.g., an agonist, antagonist, peptidomimetic, protein, peptide, nucleic acid, small molecule, or other drug candidate) to treat a disease or disorder associated with aberrant Uba6 and/or E2Z expression or activity. For example, such methods can be used to determine whether a subject can be effectively treated with an agent for cancer. Thus, the present invention provides methods for determining whether a subject can be effectively treated with an agent for a disorder associated with aberrant Uba6 and/or E2Z expression or activity in which a test sample is obtained and Uba6 and/or E2Z expression and/or Uba6 and/or E2Z activity is detected.

The methods of the invention can also be used to detect genetic alterations in a Uba6 and/or an E2Z gene, thereby determining if a subject with the altered gene is at risk for a disorder characterized by misregulation in Uba6 and/or E2Z activity, such as cancer. In certain embodiments, the methods include detecting, in a sample of cells from the subject, the presence or absence of a genetic alteration characterized by at least one of: an alteration affecting the integrity of a gene encoding a Uba6 and/or E2Z protein; the misexpression of the Uba6 and/or E2Z gene; or aberrant activity of the Uba6 and/or E2Z protein. For example, such genetic alterations can be detected by ascertaining the existence of at least one of: 1) a deletion of one or more nucleotides from a Uba6 and/or an E2Z gene; 2) an addition of one or more nucleotides to a Uba6 and/or an E2Z gene; 3) a substitution of one or more nucleotides of a Uba6 and/or an E2Z gene, 4) a chromosomal rearrangement of a Uba6 and/or an E2Z gene; 5) an alteration in the level of a messenger RNA transcript of a Uba6 and/or an E2Z gene; 6) aberrant modification of a Uba6 and/or an E2Z gene, such as of the methylation pattern of the genomic DNA; 7) the presence of a non-wild type splicing pattern of a messenger RNA transcript of a Uba6 and/or an E2Z gene; 8) a non-wild type level of a Uba6 and/or an E2Z protein; 9) allelic loss of a Uba6 and/or an E2Z gene; and 10) inappropriate Uba6 and/or E2Z activity. As described herein, there are a large number of assays known in the art which can be used for detecting alterations in a Uba6 and/or an E2Z gene.

In certain embodiments, detection of the alteration involves the use of a probe/primer in a polymerase chain reaction (PCR) (see, e.g., U.S. Pat. Nos. 4,683,195 and 4,683,202), such as anchor PCR or RACE PCR, or, alternatively, in a ligation chain reaction (LCR) (see, e.g., Landegran et al. (1988) Science 241:1077; and Nakazawa et al. (1994) Proc. Natl. Acad. Sci. USA 91:360), the latter of which can be particularly useful for detecting point mutations in the Uba6 gene (see Abravaya et al. (1995) Nucleic Acids Res. 23:675). This method can include the steps of collecting a sample of cells from a subject, isolating nucleic acid (e.g., genomic, mRNA or both) from the cells of the sample, contacting the nucleic acid sample with one or more primers which specifically hybridize to a Uba6 gene under conditions such that hybridization and amplification of the Uba6 gene (if present) occurs, and detecting the presence or absence of an amplification product, or detecting the size of the amplification product and comparing the length to a control sample. It is anticipated that PCR and/or LCR may be desirable to use as a preliminary amplification step in conjunction with any of the techniques used for detecting mutations described herein.

Alternative amplification methods include: self sustained sequence replication (Guatelli et al. (1990) Proc. Natl. Acad. Sci. USA 87:1874), transcriptional amplification system (Kwoh et al. (1989) Proc. Natl. Acad. Sci. USA 86:1173), Q-Beta Replicase (Lizardi, P. M. et al. (1988) Biotechnology 6:1197), or any other nucleic acid amplification method, followed by the detection of the amplified molecules using techniques well known to those of skill in the art. These detection schemes are especially useful for the detection of nucleic acid molecules if such molecules are present in very low numbers.

In an alternative embodiment, mutations in a Uba6 gene from a sample cell can be identified by alterations in restriction enzyme cleavage patterns. For example, sample and control DNA is isolated, amplified (optionally), digested with one or more restriction endonucleases, and fragment length sizes are determined by gel electrophoresis and compared. Differences in fragment length sizes between sample and control DNA indicates mutations in the sample DNA. Moreover, the use of sequence specific ribozymes (see, for example, U.S. Pat. No. 5,498,531) can be used to score for the presence of specific mutations by development or loss of a ribozyme cleavage site.

In other embodiments, genetic mutations in Uba6 can be identified by hybridizing a sample and control nucleic acids, e.g., DNA or RNA, to high density arrays containing hundreds or thousands of oligonucleotides probes (Cronin, M. T. et al. (1996) Human Mutation 7: 244; Kozal, M. J. et al. (1996) Nature Medicine 2:753). For example, genetic mutations in Uba6 can be identified in two dimensional arrays containing light-generated DNA probes as described in Cronin et al., supra. Briefly, a first hybridization array of probes can be used to scan through long stretches of DNA in a sample and control to identify base changes between the sequences by making linear arrays of sequential overlapping probes. This step allows the identification of point mutations. This step is followed by a second hybridization array that allows the characterization of specific mutations by using smaller, specialized probe arrays complementary to all variants or mutations detected. Each mutation array is composed of parallel probe sets, one complementary to the wild-type gene and the other complementary to the mutant gene.

In yet another embodiment, any of a variety of sequencing reactions known in the art can be used to directly sequence the Uba6 and/or E2Z gene and detect mutations by comparing the sequence of the sample Uba6 and/or E2Z with the corresponding wild-type (control) sequence. Examples of sequencing reactions include those based on techniques developed by Maxam and Gilbert ((1977) Proc. Natl. Acad. Sci. USA 74:560) or Sanger ((1977) Proc. Natl. Acad. Sci. USA 74:5463). It is also contemplated that any of a variety of automated sequencing procedures can be utilized when performing the diagnostic assays ((1995) Biotechniques 19:448), including sequencing by mass spectrometry (see, e.g., PCT International Publication No. WO 94/16101; Cohen et al. (1996) Adv. Chromatogr. 36:127-162; and Griffin et al. (1993) Appl. Biochem. Biotechnol. 38:147).

Other methods for detecting mutations in the Uba6 and/or E2Z gene include methods in which protection from cleavage agents is used to detect mismatched bases in RNA/RNA or RNA/DNA heteroduplexes (Myers et al. (1985) Science 230:1242). In general, the art technique of “mismatch cleavage” starts by providing heteroduplexes formed by hybridizing (labeled) RNA or DNA containing the wild-type Uba6 and/or E2Z sequence with potentially mutant RNA or DNA obtained from a tissue sample. The double-stranded duplexes are treated with an agent which cleaves single-stranded regions of the duplex such as which will exist due to base pair mismatches between the control and sample strands. For instance, RNA/DNA duplexes can be treated with RNase and DNA/DNA hybrids treated with S1 nuclease to enzymatically digesting the mismatched regions. In other embodiments, either DNA/DNA or RNA/DNA duplexes can be treated with hydroxylamine or osmium tetroxide and with piperidine in order to digest mismatched regions. After digestion of the mismatched regions, the resulting material is then separated by size on denaturing polyacrylamide gels to determine the site of mutation. See, for example, Cotton et al. (1988) Proc. Natl. Acad. Sci. USA 85:4397; Saleeba et al. (1992) Methods Enzymol. 217:286. In an exemplary embodiment, the control DNA or RNA can be labeled for detection.

In still another embodiment, the mismatch cleavage reaction employs one or more proteins that recognize mismatched base pairs in double-stranded DNA (so called “DNA mismatch repair” enzymes) in defined systems for detecting and mapping point mutations in Uba6 and/or E2Z cDNAs obtained from samples of cells. For example, the mutY enzyme of E. coli cleaves A at G/A mismatches and the thymidine DNA glycosylase from HeLa cells cleaves T at G/T mismatches (Hsu et al. (1994) Carcinogenesis 15:1657). According to an exemplary embodiment, a probe based on a Uba6 and/or an E2Z sequence, e.g., a wild-type Uba6 and/or E2Z sequence, is hybridized to a cDNA or other DNA product from a test cell(s). The duplex is treated with a DNA mismatch repair enzyme, and the cleavage products, if any, can be detected from electrophoresis protocols or the like. See, for example, U.S. Pat. No. 5,459,039.

In other embodiments, alterations in electrophoretic mobility will be used to identify mutations in Uba6 and/or E2Z genes. For example, single strand conformation polymorphism (SSCP) may be used to detect differences in electrophoretic mobility between mutant and wild type nucleic acids (Orita et al. (1989) Proc. Natl. Acad. Sci. USA 86:2766, see also Cotton (1993) Mutat. Res. 285:125; and Hayashi (1992) Genet. Anal Tech. Appl. 9:73). Single-stranded DNA fragments of sample and control Uba6 nucleic acids will be denatured and allowed to renature. The secondary structure of single-stranded nucleic acids varies according to sequence, the resulting alteration in electrophoretic mobility enables the detection of even a single base change. The DNA fragments may be labeled or detected with labeled probes. The sensitivity of the assay may be enhanced by using RNA (rather than DNA), in which the secondary structure is more sensitive to a change in sequence. In an exemplary embodiment, the subject method utilizes heteroduplex analysis to separate double stranded heteroduplex molecules on the basis of changes in electrophoretic mobility (Keen et al. (1991) Trends Genet. 7:5).

In yet another embodiment, the movement of mutant or wild-type fragments in polyacrylamide gels containing a gradient of denaturant is assayed using denaturing gradient gel electrophoresis (DGGE) (Myers et al. (1985) Nature 313:495). When DGGE is used as the method of analysis, DNA will be modified to insure that it does not completely denature, for example by adding a GC clamp of approximately 40 bp of high-melting GC-rich DNA by PCR. In a further embodiment, a temperature gradient is used in place of a denaturing gradient to identify differences in the mobility of control and sample DNA (Rosenbaum and Reissner (1987) Biophys. Chem. 265:12753).

Examples of other techniques for detecting point mutations include, but are not limited to, selective oligonucleotide hybridization, selective amplification, or selective primer extension. For example, oligonucleotide primers may be prepared in which the known mutation is placed centrally and then hybridized to target DNA under conditions which permit hybridization only if a perfect match is found (Saiki et al. (1986) Nature 324:163); Saiki et al. (1989) Proc. Natl. Acad. Sci. USA 86:6230). Such allele specific oligonucleotides are hybridized to PCR amplified target DNA or a number of different mutations when the oligonucleotides are attached to the hybridizing membrane and hybridized with labeled target DNA.

Alternatively, allele specific amplification technology which depends on selective PCR amplification may be used in conjunction with the instant invention. Oligonucleotides used as primers for specific amplification may carry the mutation of interest in the center of the molecule (so that amplification depends on differential hybridization) (Gibbs et al. (1989) Nucl. Acids Res. 17:2437) or at the extreme 3′ end of one primer where, under appropriate conditions, mismatch can prevent, or reduce polymerase extension (Prossner (1993) Tibtech 11:238). In addition it may be desirable to introduce a novel restriction site in the region of the mutation to create cleavage-based detection (Gasparini et al. (1992) Mol. Cell. Probes 6: 1). It is anticipated that in certain embodiments amplification may also be performed using Taq ligase for amplification (Barany (1991) Proc. Natl. Acad. Sci. USA 88:189). In such cases, ligation will occur only if there is a perfect match at the 3′ end of the 5′ sequence making it possible to detect the presence of a known mutation at a specific site by looking for the presence or absence of amplification.

The methods described herein may be performed, for example, by utilizing pre-packaged diagnostic kits comprising at least one probe nucleic acid or antibody reagent described herein, which may be conveniently used, e.g., in clinical settings to diagnose patients exhibiting symptoms or family history of a disease or illness involving a Uba6 gene. Furthermore, any cell type or tissue in which Uba6 is expressed may be utilized in the prognostic assays described herein.

Monitoring of Effects During Clinical Trials

Monitoring the influence of agents (e.g., drugs) on the expression or one or more activities of Uba6 and/or E2Z (e.g., ubiquitin activation; ubiquitin-adenylate intermediate formation; ubiquitin thiol esterification; ubiquitin transfer to one or more ubiquitin-conjugating enzymes (e.g., E2s); ubiquitin transfer to one or more ubiquitin-protein ligases (e.g., E3s); and/or ubiquitination of one or more target polypeptides (e.g., ubiquitin, an E2, an E3, a target polypeptide and/or protein or the like) can be applied not only in basic drug screening, but also in clinical trials. For example, the effectiveness of an agent determined by a screening assay as described herein to increase Uba6 and/or E2Z gene expression or protein levels, or upregulate Uba6 and/or E2Z activity, can be monitored in clinical trials of subjects exhibiting decreased Uba6 and/or E2Z gene expression, protein levels, or downregulated Uba6 and/or E2Z activity. Alternatively, the effectiveness of an agent determined by a screening assay to decrease Uba6 and/or E2Z gene expression, or protein levels, or downregulate or Uba6 and/or E2Z activity, can be monitored in clinical trials of subjects exhibiting increased Uba6 and/or E2Z gene expression, protein levels, or upregulated Uba6 and/or E2Z activity.

In an exemplary embodiment, the present invention provides a method for monitoring the effectiveness of treatment of a subject with an agent (e.g., an agonist, antagonist, peptidomimetic, protein, peptide, nucleic acid, small molecule, or other drug candidate identified by the screening assays described herein) including the steps of (i) obtaining a pre-administration sample from a subject prior to administration of the agent; (ii) detecting the level of expression of a Uba6 and/or an E2Z protein, mRNA, or genomic DNA or of the level of Uba6 and/or E2Z activity in the preadministration sample; (iii) obtaining one or more post-administration samples from the subject; (iv) detecting the level of expression or activity of the Uba6 and/or E2Z protein, mRNA, or genomic DNA or of the level of Uba6 and/or E2Z activity in the post-administration samples; (v) comparing the level of expression or activity of the Uba6 and/or E2Z protein, mRNA, or genomic DNA or of the level of Uba6 and/or E2Z activity in the pre-administration sample with the Uba6 and/or E2Z protein, mRNA, or genomic DNA or Uba6 and/or E2Z activity in the post administration sample or samples; and (vi) altering the administration of the agent to the subject accordingly. For example, increased administration of the agent may be desirable to increase the expression or activity of Uba6 and/or E2Z to higher levels than detected, i.e., to increase the effectiveness of the agent. Alternatively, decreased administration of the agent may be desirable to decrease expression or activity of Uba6 to lower levels than detected, i.e. to decrease the effectiveness of the agent. According to such an embodiment, Uba6 and/or E2Z expression or activity may be used as an indicator of the effectiveness of an agent, even in the absence of an observable phenotypic response.

This invention is further illustrated by the following examples, which should not be construed as limiting. The contents of all references, patents and published patent applications cited throughout this application are hereby incorporated by reference in their entirety for all purposes.

Example I A Novel Vertebrate E1 Protein, Uba6, Promotes Activation of Ubiquitin In Vitro

The E1 for ubiquitin is a member of a family of proteins that contains a protein fold, the ThiF domain, found in ancient metabolic enzymes in bacteria (Lake et al. (2001) Nature 414:325). Using this domain to identify related proteins bioinformatically, a previously uncharacterized protein was identified with similarity to Ube1, the canonical E1 activating enzyme for ubiquitin (GenBank Accession FLJ10808), Uba6. Uba6 orthologs exist in mouse, rat, and zebrafish (Dr), but are not found in non-vertebrate metazoans or in yeast (FIG. 1A). Phylogenetically, Uba6 is more distantly related to Ube1 than is Ube1L (FIG. 1G).

The ability of recombinant Uba6 (expressed in and purified from insect cells, FIG. 1B) to activate several ubiquitin like proteins as well as ubiquitin itself was examined. Surprisingly, it was determined that Uba6 could activate ubiquitin, but not any of the other ubiquitin like proteins tested, as determined by the appearance of an Uba6˜Ub thiol ester upon non-reducing SDS-PAGE analysis (FIG. 1C, lane 4). As expected, the formation of Uba6˜Ub could be reversed by the addition of DTT, indicative of the presence of a thiol ester (FIG. 1D, lanes 5 and 6). The ability of Uba6 to promote ubiquitin activation appeared to be specific, as Ube1L, which is more closely related to Ube1 than is Uba6 (FIG. 1A) was not capable of activating ubiquitin but was able to activate its known target, ISG15 (FIG. 1E). Taken together, these data indicate that Uba6 can promote the charging of ubiquitin but not a number of other ubiquitin-like proteins.

The identification of Uba6 as an activating enzyme for ubiquitin was unexpected and therefore several experiments described below were performed to validate this conclusion. Initially, the classical approaches of Hershko (Hershko et al. (1983) J. Biol. Chem. 258:8206) were used to purify Ube1 to examine whether Uba6 could be similarly purified. Extracts were made from 293T cells stably expressing Flag-HA-Uba6 and were incubated with ubiquitin-agarose in the presence of ATP. Beads were subsequently washed with buffer lacking DTT and proteins then eluted with buffer containing DTT, which reverses thiol esters between E1s and ubiquitin or ubiquitin-like proteins. Load, flow-through, washes and eluates, as well as proteins remaining associated with the ubiquitin-agarose beads, were then subjected to immunoblotting with anti-Ube1 to detect Ube1 and anti-HA to detect Flag-HA-Uba6. The elution pattern of Uba6 closely paralleled that of Ube1 (FIG. 1F). In particular, Uba6 was eluted specifically upon treatment with DTT. These data indicate that, like Ube1, Uba6 can be chemically crosslinked to ubiquitin in a thiol-dependent manner.

Example II Identification of Uba6 Conjugated to Ubiquitin In Vivo

In order to validate the association between Uba6 and ubiquitin in vivo, cell lines were developed that stably expressed three forms of Uba6: 1) the wild-type protein containing an N-terminal Flag-HA tag; 2) the C625A mutant in which the catalytic cysteine was mutated to alanine, also as an N-terminal Flag-HA fusion; and 3) the C625S mutant in which the catalytic cysteine was mutated to serine, also as an N-terminal Flag-HA fusion. Replacement of the active site cysteine with alanine was expected to inactivate the ability of the protein to promote ubiquitin activation. In contrast, the cysteine to serine mutant was expected to support formation of an ester linked ubiquitin or ubiquitin like protein which is more stable that the thiol ester formed with cysteine. Indeed, previous studies (Komatsu et al. (2004) Embo J. 23:1977) have demonstrated that in certain cases, more stable conjugates can be formed with ester linkages than with thiol ester linkages.

All three proteins were expressed at comparable levels upon analysis in SDS-PAGE in the absence of reducing agents (FIG. 2A). While the wild-type protein and the C625A mutant migrated at identical positions, a large fraction of the C625S mutant protein displayed reduced mobility by SDS-PAGE, as observed by both immunoblotting with anti-HA antibodies and by staining the membrane with Ponceau stain (FIG. 2A). These data indicate that Uba6 forms a stable ester with ubiquitin or a ubiquitin-like protein via S625 in this mutant protein. To examine whether the slow mobility form of Uba6 might be carrying ubiquitin or the ubiquitin-like protein Nedd8, an immunoblot was performed using anti-ubiquitin or anti-Nedd8 antibodies (FIG. 2A). While ubiquitin was detected in the slower mobility form of Uba6 when C625 was replaced by serine, no ubiquitin was found with either the wild-type protein or the C625A mutant protein, indicating that the more slowly migrating form of the Uba6^(C625S) protein contained ubiquitin. In contrast, Nedd8 was not detected, although Nedd8 was clearly detected in an SCF complex used as a positive control (FIG. 2A).

Next, additional confirmation that Uba6^(C625S) was conjugated to ubiquitin was sought. Preparative samples of Flag-HA-Uba6^(C625S) were purified from 293T cells stably expressing Flag-HA-Uba6^(C625S) under control of the CMV promoter. Approximately 30% of the isolated protein was determined to migrate at a position consistent with its modification by a ubiquitin-like protein (FIG. 3C). Mass spectral analysis of the more slowly migrating protein identified three peptides from ubiquitin (FIG. 3C), consistent with Uba6^(C625S) forming conjugates with ubiquitin. In contrast, the more rapidly migrating form of the protein lacked peptides for ubiquitin. Neither form of Uba6^(C625S) contained peptides for any other ubiquitin-like protein, indicating that other ubiquitin-like proteins were not able to be activated by Uba6 under these conditions.

To rule out any effects of overexpression on these results, Flag-HA-Uba6, Flag-HA-Uba6^(C625A) and Flag-HA-Uba6^(C625S) were expressed at near endogenous levels from a lentiviral vector driven by the PGK promoter. Again, the C625S mutant, but not WT or C625A proteins, expressed a more slowly migrating protein consistent with ubiquitination (FIG. 2B). Taken together, these data indicate that Uba6 functions to activate ubiquitin in tissue culture cells.

Example III Uba6 Promotes Transfer of Ubiquitin to E2s with Specificity Distinct from Ube1

Ube1 promotes the transfer of ubiquitin from its active site cysteine residue to the active site cysteine of an E2 ubiquitin conjugating enzyme. A subset of E2s are known to be activated by Ube1, although a small number of E2s are specific for transfer to ubiquitin-like proteins (E2M/Ubc12 is charged by Nedd8, E21/Ubc9 is charged by SUMO, and E2L/Ubc8 is charged by ISG15). To determine whether Uba6 was capable of transferring ubiquitin to one or more E2s, more than two dozen E2s found in the human genome were cloned and in vitro charging assays were performed using non-reducing SDS-PAGE. In this assay, charged E2s migrated more slowly than the uncharged E2s. Examples of charging assays are shown in FIG. 3A and the data for all of the E2s studies is collected in FIG. 3B. It was discovered that the specificity of Ube1 and Uba6 for these E2s was distinct. Neither Ube1 or Uba6 promoted charging of E2M, E2M3, HSPC, E2L6, E2U or E21, as expected. Ube1 selectively promoted charging of E2J1, E2J2, E2K, E2Q2, E2R1/Cdc34, E2R2/Cdc34B and E2W, while both Ube1 and Uba6 promoted charging of E2C, E2D1, E2D2, E2D3, E2D4, E2E1, E2G, E2S and E2T, to varying degrees. Interestingly, Uba6 but not Ube1 promoted charging of E2Z, a previously uncharacterized E2 of unknown function. Without intending to be bound by theory, these data indicate that Ube1 and Uba6 have distinct substrate specificities with respect to the E2s they charge and, therefore, may function in distinct pathways. Without intending to be bound by theory, Uba6 may function with non-canonical E2s (i.e., E2s that cannot be identified through sequence relationships with the known E2s). Methods known in the art will be used to easily identify such proteins.

Example IV The Ub1 Domain of Ube1 and Uba6 Control their Substrate Specificities

Previous studies with Uba3, a subunit of the E1 for Nedd8, have revealed that its C-terminal ubiquitin fold is involved in recognizing its specific E2, Ubc12 (Huang et al. (2004) Nat. Struct. Mol. Biol. 11:927). While the ThiF and catalytic domains of Ube1 and Uba6 are 43% identical, their C-terminal ubiquitin-like domains are more distantly related (˜33% identical) (FIGS. 4A, B). In contrast, the Ub1 domains in Uba6 are very highly conserved in Uba6 proteins from zebrafish to human (˜75%) (FIG. 1G), indicating selective pressure to maintain sequences in the C-terminal Ub1 domain of Uba6. To examine whether substrate selectivity of Uba6 relative to Uba1 may reflect its Ub1 domain, a Ub1-domain swap experiment was performed (FIG. 4C). The Ub1 domain of Ube1 was replaced with the Ub1 domain from Uba6 and vice versa. While Ube1 could activate Cdc34/E2R1, Uba6 was defective in this activity. However, Uba6 carrying the Ub1 domain of Ube1 was active in Cdc34 charging. Consistent with this, Ube1 containing the Ub1 domain from Uba6 was defective in Cdc34 charging. Both Ube1 and Uba6 lacking their Ub1 domains were defective for Cdc34 charging. These data indicate that the C-terminal Ub1 domains in Ube1 and Uba6 are important for the specificity of ubiquitin charging, and that the Ub1 domain of Uba6 is more discriminating than is the Ub1 domain of Ube1.

Example V A Method for Reducing the Abundance of Uba6 in Animal Cells

In order to detect Uba6 in animal cells, an antibody was developed against the C-terminal Ub1 domain. This antibody recognized a doublet of proteins at approximately 125 Kd, the expected size of Uba6, in extracts from several cell lines including 293T cells (FIG. 2D). This antibody also reacted with purified Flag-HA-Uba6, which migrates slightly slower on SDS-PAGE than does the endogenous protein due to the presence of an epitope tag. It was determined that three different siRNAs targeting Uba6 could greatly reduce its steady-state levels (FIG. 2D). These data indicates that the activity/function of Uba6 can be inhibited using RNAi.

Example VI Ubc5 Charging

An analysis of the activity of chimeric proteins (replacement of the Ub1 domain of Uba1 with the Ub1 domain of Ube1; replacement of the Ub1 domain from Ube1 with the Ub1 domain of Uba6 (FIGS. 11A and 11B)) toward Ubc5, which is charged by both Ube1 and Uba6, was performed. It was determined that, while Uba6 and Ube1 lacking the Ub1 domains were devoid of activity toward Ubc5, each of the chimeric proteins was active (FIG. 11C). These data indicate that the Ub1 domains are interchangeable with respect to this shared E2. It also indicates that the absence of activity of the chimeras towards Cdc34 or E2Z was not because the protein is generally non-functional. Importantly, the Ube1 protein containing the Uba6 Ub1 domain was still inactive toward E2Z, indicating that additional structural features outside the Ub1 domain may be required for specificity. Molecular modeling studies were also performed that indicated that the Ub1 domains from both Uba6 and Ube1 fold into ubiquitin-like folds (FIG. 11D).

Example VII Uba6 is Required to Charge E2Z In Vivo

An in vivo charging assay coupled with RNAi was used to deplete either Uba6 or Ube1. In the in vivo charging assay, cell lysates were generated using buffers at pH ˜4.5 [50 mM MES, pH 4.5, 0.5% NP40, 100 mM NaCl] and immediately subjected to non-reducing SDS-PAGE. Under these conditions, the E2 thiol esters remained relatively stable, allowing their separation by SDS-PAGE. Charged and uncharged E2s were then detected by immunoblot. Because antibodies against E2Z were not available, a cell line that stably expresses a Flag-HA-tagged version of E2Z was generated. As shown in FIG. 12A, a fraction of E2Z migrated slightly more slowly than the majority of the protein. This band was absent when a mutant form of E2Z was employed in which the active site cysteine was mutated to alanine and could not be ubiquitinated, or when DTT was added to wild-type E2Z (which will reduce the thiol-ester). As shown in FIG. 12B, using three different siRNAs to deplete Uba6, it was determined that in vivo charging of E2Z was completely dependent on Uba6. In contrast, depletion of Ube1 had no effect on the extent of charging of E2Z. Ube1 depletion did, however, reduce the extent of charging of the Ube1-specific E2 Cdc34.

Example VIII Structural and Functional Diversity of Ubiquitin E1s

Humans, chimps, old-world monkeys, and zebrafish contain only a single UBE1 gene, which, in humans, is located on the X-chromosome. However, in the mouse, there appears to have been a duplication of the X-linked UBE1 gene, as there is a second gene located on the Y-chromosome (Levy et al. (2000) Mamm. Genome 11: 164; Mitchell et al. (1992) Nature 359:528; Odorisio et al. (1996) Dev. Biol. 180:336). The mouse Ube1y protein is 90% identical to mouse Ube1x and these proteins are located on the same clad of the E1 dendogram (FIG. 20). In fact, mouse Ube1y is more closely related to mouse Ube1x and the corresponding human Ube1 protein than is the zebrafish Ube1 protein (FIG. 20). There are also UBE1 related sequences on rat chromosome-Y but these sequences do not appear to constitute a complete UBE1 gene (see GenBank ID 25225). Multiple Ube1 orthologs exist in plants (for example, 2 genes in Arabidopsis (Hatfield et al. (1997) Plant J. 11:213) and three genes in wheat, FIG. 20). Again, the encoded E1 proteins within each species are closely related to each other (˜90% identical) and form a single clad on the E1 dendogram (FIG. 20). Previous studies have failed to identify differences in the functional properties of Ube1-related E1s, indicating that these genes likely provide the same biochemical functions but in different cell types or in different developmental settings. Arabidopsis Ube1 proteins have identical activities towards all E2s tested (Hatfield et al. (1997) Plant J. 11:213). In addition, it was previously concluded that mouse Ube1y is expressed exclusively in the testes and its induction during spermatogenesis is thought to reflect a requirement for increased levels of conventional ubiquitin-activating enzyme activity during this stage of development where the Ube1x protein is also expressed (Odorisio et al. (1996) Dev. Biol. 180:336). In contrast, human, mouse and zebrafish Uba6 genes are more closely related to each other and distantly related to the Ube1 clad (FIG. 20). Indeed, the Uba6 proteins form their own clad, consistent with the finding that human Uba6 and Ube1 play distinct roles in E2 charging in vivo (FIG. 15).

In contrast to C. elegans, D. melanogaster, S. pombe, and S. cerevisiae, which appear to lack Uba6 orthologs, both Ube1 and Uba6 orthologs were identified in sea urchin (S. purpuratus) (XP_(—)795302, 65% identity and XP_(—)780782, 56% identity over 709 residues, respectively).

Example IX Specificity Elements in the E1^(Ufd)-E2 Interface

The results presented herein indicate that Ube1 and Uba6 have the capability of interacting and charging different sets of E2 conjugating enzymes and based on deletion analysis and the analysis of chimeric proteins. The results presented herein also indicate that the C-terminal Ubiquitin-fold domain (Ufd) of Uba6 and Ube1 contribute to recruitment and/or transthiolation of E2s (FIG. 14). Currently, there are no structures available for Ube1, Uba6, Use1 or Cdc34. Models of these proteins were built based on the structures of related proteins: the Ufd from SUMO1 (pdb code: 1Y8Q) for Uba6 and Ube1, UbcH1 (1TTE) in the case of Cdc34A, and UbCH5B (2ESQ) in the case of Use1. Models for Ufds and Cdc34 were generated using Modeller while models for Use1 were generated using Swissmodel. These models were then compared with the structure of the Uba3^(Ufd)-Ucb12 complex (1Y8X) (Huang et al. (2005) Mol. Cell. 17:341) in Pymol.

In the Ubc12/Uba3^(Ufd) structure (FIG. 21B), the N-terminal helix (Helix 1) of Ubc12 makes several contacts with the surface of a β-sheet in the Ufd composed primarily of S2, S3, and S4 (FIG. 21B). The surface of the Ufd β-sheet facing the Ubc12 helix is composed primarily of residues with small and/or hydrophilic side chains (A380, T382, T384, T391, A424, A426) (FIGS. 21A and 21B). This facilitates interaction with Q31, L32, Q35, and N39 in the Ubc12 helix. R390 in the Uba3^(Ufd) protrudes from the opposite side of the β-sheet and makes contacts with D427 in the L2 loop of Ubc12 (FIG. 21B). Structure-based alignment of Uba6 and Ube1 with the SUMO1 Ufd fold, and Use1 and Cdc34A with the UBC fold provides models of Uba6^(Ufd)-Use1 and Ube1^(Ufd) Cdc34 complex (FIGS. 21C and 21D). Although it is not possible to directly discern the structural basis for specificity from these models, it is possible to identify differences between Ufd and UBC sequences within the presumptive interface which may participate in specificity. First, the β-sheet surfaces of the three Ufds have distinct charge distributions (FIG. 21D). The Uba3^(Ufd) is the most basic (blue), followed by the Ube1^(Ufd) and the Uba6^(Ufd). Moreover, the Uba3^(Ufd) has an acidic patch (red) not seen in the Ufds from Uba6 or Ube1. In contrast to the H1 helix of Ubc12, the H1 helix of Use1 is much more hydrophobic in character, consistent with the more hydrophobic character of the predicted S2 and S3 β-sheets in the Uba6^(Ufd) compared with the Uba3^(Ufd) (FIGS. 21A and 21C). The L2 loop of Use1 likely makes interactions with the S3 and H2 in the Uba6^(Ufd) but uncertainties in the structural prediction make it impossible to assess these potential interactions. The predicted structure of the Ube1^(Ufd) contains a poorly modeled segment which clashes with H1 of Cdc34 when superimposed on the Uba3^(Ufd)-Ubc12 structure (FIG. 21D). Nevertheless, it is possible to identify both similarities and differences in the interface residues that may make contacts in the respective interfaces. For example, R1029 at the end of S4 in Ube1 corresponds to D1023 in Uba6 and A426 in Uba3 (FIG. 21A). In addition, L997 in S2 of Ube1 corresponding to V986 in Uba6 and T384 in Uba3 (FIG. 21A). Likewise, significant differences exist in the residues in H1 of the respective specific E2s (FIG. 21A).

Although differences in residues predicted to be at the interface of the E2 and the Ufd for the complexes analyzed here are evident, without intending to be bound by theory, it is believed, based on the data presented herein, that specificity in this system is unlikely to reflect one or a small number of changes in amino acids at the interface between E2 and Ufd. First, an in-depth analysis of E2 sequences (especially the N-terminal helix which binds Ufds) failed to identify classes of residues that might segregate E2s into distinct classes related to the E1s that they function with (Winn et al. (2004) Structure 12:1563). Second, extensive alanine scanning mutagenesis of the Ubc12/Uba3-Ufd interaction indicates that many residues will contribute to the interaction. For example, mutation of 8 of 9 interface residues in Ubc12 reduced or abolished its charging by Nedd8 (Huang et al. (2005) Mol. Cell. 17:341; Huang et al. (2007) Nature 445:394). Likewise, mutation of 4 of 5 residues on the interaction surface of Uba3's Ufd reduced charging of Ubc12 (Huang et al. (2005) Mol. Cell. 17:341). Thus, structural analysis will likely be required to address this important and interesting question.

Example X Materials and Methods Plasmids

Open reading frames for Ube1, Uba6, E2 ubiquitin conjugating enzymes, and ubiquitin-like proteins were amplified by PCR from either cDNA templates or cDNA libraries and cloned into pENTR/D-Topo (Invitrogen, Carlsbad, Calif.). These open reading frames were then transferred to the indicated destination plasmids using recombination-mediated cloning with CLONASE™ (Invitrogen, Carlsbad, Calif.). Mutations were created using PCR-based mutagenesis. All open reading frames were sequenced in their entirety. Baculoviruses expressing GST-Ube1L and GST-MP1 were provided by Brenda Schulman (St. Jude Children's Research Hospital, Memphis, Tenn.).

Unless otherwise noted, open reading frames were cloned into pENTR/TOPO (Invitrogen, Carlsbad, Calif.) and transferred into the appropriate expression plasmid using in vitro recombination with Clonase (Invitrogen). The pHAGE-Flag-HA vector (puromycin resistant) places the open reading frame under control of the PGK promoter. Open reading frames for E2s were placed into vectors containing T7 promoters and an N-terminal His-6 tag. For expression of Use1 in bacteria, the Use1 open reading frame (NM_(—)023079) was cloned into pENTR-2 containing a TEV protease cleavage site upstream of the open reading frame and transferred into pDEST-15 (N-terminal GST tag from Invitrogen). The annotated open reading frame for Use1 (referred to as UBE2Z) (Gu, X. et al. Cloning and characterization of a gene encoding the human putative ubiquitin conjugating enzyme E2Z (UBE2Z). Mol Biol Rep (2006)) is incorrect, as determined by the size of the endogenous protein detected using anti-Use1 antibodies and by DNA sequence analysis. The actual open reading frame initiates 109 amino acids prior to the annotated start site (FIG. 20B). The sequences of all the genes examined are collected in FIG. 22.

mRNA Expression

Analysis of mRNA expression for Uba6, Ube1, and Use1 was performed using the Genomics Institute of the Novartis Research Foundation transcriptional profiling resource (Su et al. (2002) Proc. Natl. Acad. Sci. U.S.A. 99:4465; Su et al. (2004) Proc. Natl. Acad. Sci. U.S.A. 101:6062). This analysis employed GNF1H chips in combination with the MAS5 transcriptional profiling algorithm (Su et al. (2004) Proc. Natl. Acad. Sci. U.S.A. 101:6062).

Phylogenetic Trees

Trees and alignments were generated using ClustalW in conjunction with Treeview.

Antibodies

Uba6 and Use1 antibodies were generated in rabbits using a GST-Uba6 fusion protein (residues 869-1052) and GST-Use1, respectively, made in bacteria. Antibodies were affinity purified prior to use. The specificity of antibodies was determined by RNAi against Uba6 Use1.

Protein Expression and Purification

For production of proteins in insect cells, recombinant baculoviruses were used to infect Sf9 cells (40 h) and cleared cell extracts in lysis buffer (50 mM Tris-HCl, pH 7.5, 0.5 mM DTT, 150 mM NaCl, 0.5% Nonidet-P40, protease inhibitors (Roche)) were bound to anti-Flag or GSH-Sepharose beads. Washed beads were eluted with Flag peptide (500 μg/ml) or with glutathione (40 mM) and the protein dialyzed against 50 mM Tris-HCl, pH 7.5, 0.5 mM DTT, 150 mM NaCl, 50% glycerol. Bacterial expression was performed in Rosetta/DE3 cells using 0.4 mM IPTG induction for 3 h at 37° C. Cells were disrupted in lysis buffer prior to purification using GSH-Sepharose. All GST-UBL proteins were found to contain the appropriate C-termini by mass spectrometry.

Unless otherwise noted, expression of proteins in mammalian cells was accomplished by viral transduction using Gateway-compatible pHAGE (lentiviral) based vectors. Vectors were packaged in 293T cells and used to transduce the indicated cell line prior to selection with puromycin. For immunoprecipitation, extracts were generated in lysis buffer prior to incubation with the indicated antibodies. Proteins were separated on 4-12% Tris-glycine gradient gels (Invitrogen). To examine E2 charging, cells were lysed in 50 mM MES, 150 mM NaCl, 0.2% Nonidet-P40, protease inhibitors (Roche, Indianapolis, Ind.) (pH 3-5 as indicated) buffer and cleared by centrifugation. Ten μg of extract was subjected to non-reducing Novex 4-12% Bis-Tris gel prior to immunoblotting. In cases where RNAi was employed, cells were transfected with the indicated siRNAs (33 nM) using Oligofectamine (Invitrogen). After 72 hours, cells were lysed as described above to examine E2 charging. The sequences of siRNAs are provided herein. In order to examine thioesters between ubiquitin and Flag-HA-Uba6, cells expressing Flag-HA-Uba6 (wild-type or the C625A mutant) were lysed in 50 mM MES, pH 4.5, 150 mM NaCl, 0.2% Nonidet-P40, protease inhibitors and extracts subjected to immunoprecipitation with anti-HA-agarose. Immune complexes were washed with lysis buffer (pH 4.5), eluted with HA peptide, and then subjected to electrophoresis on a 412% Bis-Tris gel in the absence of reducing agent. In some cases, samples were treated with 200 mM DTT prior to electrophoresis in order to reduce thioester bonds. Gels were transferred to PVDF and probed with the indicated antibodies or, in some cases, bands were excised and subjected to mass spectrometry as described below.

In Vitro Ubiquitin Activation and E2 Charging Assays

For ubiquitin activation, the indicated E1 (8 nM) was incubated with 100 nM GSTUBL and 2 mM ATP in reaction buffer (50 mM Tris-HCl, 5 mM KCl, 5 mM MgCl₂) for 15 minutes at 30° C. (total volume 10 μl), the reaction quenched by addition of 2× Laemli sample buffer lacking reducing agent, and immediately subjected to non-reducing 4-12% Tris-glycine gel and immunoblotting. To measure the kinetics of ubiquitin activation, reactions were performed with 8 nM E1 and 0.5 μM ubiquitin for the indicated time at 30° C. and immunoblots quantified using CCD detection of chemiluminescence. This was accomplished using an Alpha Innotech FlourChem 8900 instrument. In experiments where the dependent on GST-ubiquitin concentration was determined, the GST-ubiquitin concentrations were 0, 7.5, 15, 30, 60, 120, 240, and 480 nM and the reaction time was 10 minutes. The average reaction rates for 2 independent experiments are shown together with the standard deviation. To examine E2 charging, the indicated E2 was in vitro translated using a bacterial S30 extract (Promega) in the presence of ³⁵S-methionine. Bacterial S30 extracts lack ubiquitin activating and conjugating enzymes which could interfere with he assays. Radiolabeled E2 (1 μl) was incubated with 40 nM of the indicated E1, 25 μM lysine-free ubiquitin (Boston Biochem, Waltham Mass.), 2 mM ATP in reaction buffer (15 min, 30° C.) (10 μl total volume). Reactions were analyzed as described for ubiquitin activation. In some experiments, Use1 purified from bacteria was employed. Ube1 was released from GST-TEV-Use1 using TEV protease and the eluted Use1 dialyzed against 50 mM Tris-HCl, pH 7.5, 0.5 mM DTT, 150 mM NaCl, 50% glycerol. Capture of Uba6 and Ube1 using ubiquitin-Sepharose was performed as described previously (Pickart et al. (1985) J. Biol. Chem. 260:1573; Ciechanover et al. (1982) J. Biol. Chem. 257:2537) using extracts from cells stably expressing Flag-HA-Uba6.

Mass Spectroscopy

Unless otherwise stated, mass spectrometry was performed on peptides produced by in-gel trypsinization using a Thermo-Electron LTQ mass spectrometer. Searches were performed using Sequest. For determination of C-termini in GST-UBL fusion proteins, 2 μg of protein was excised from SDS-PAGE gels and digested with the protease indicated. Digested peptides were then subjected to LC/MS/MS under conditions where >3250 MS/MS scans were obtained for each LC/MS/MS run. The predicted C-terminal peptides as well as XCorr and dCn scores for the match with the predicted spectra. For analysis of peptides from Flag-HA-Uba6˜Ub purified from mammalian cells, trypsinized samples were subjected to LC/MS/MS using an LTQ-Orbitrap instrument to identify peptides with high mass accuracy. Tandem mass spectra were first searched using Sequest against the human genome, identifying peptides against ubiquitin and Uba6 but not other ubiquitin like proteins. In order to more effectively rule out the presence of additional UBLs, a focused database containing protein sequences for human ubiquitin, Nedd8, SUMO-1, SUMO-2, SUMO-3, Fub1, Fat10, Urm1, Ufm1, Gdx, Isg15, and Apg12 was searched using a 100 mass unit filter (facilitated by the high mass accuracy afforded by orbitrap detection). Peptides corresponding to only ubiquitin were obtained (see FIG. 2), making it extremely unlikely that Uba6 efficiently charges UBLs in addition to ubiquitin.

For production of proteins in insect cells, recombinant baculoviruses were used to infect Sf9 cells (40 hours) and cleared cell extracts in lysis buffer (50 mM Tris-HCl, pH 7.5, 0.5 mM DTT, 150 mM NaCl, 0.5% Nonidet-P40, protease inhibitors (Roche)) were bound to anti-Flag or GSH-Sepharose beads. Washed beads were eluted with Flag peptide (500 μg/ml) or with glutathione (40 mM) and the protein dialyzed against 50 mM Tris-HCl, pH 7.5, 0.5 mM DTT, 150 mM NaCl, 50% glycerol. Bacterial expression was performed in Rosetta/DE3 cells using 0.4 mM IPTG induction for 3 h at 37° C. Cells were disrupted in lysis buffer prior to purification using GSH-Sepharose. All GST-UBL proteins were found to contain the appropriate C-termini by mass spectrometry.

Unless otherwise noted, expression of proteins in mammalian cells was accomplished by viral transduction using Gateway-compatible pHAGE (lentiviral) based vectors. Vectors were packaged in 293T cells and used to transduce the indicated cell line prior to selection with puromycin. For immunoprecipitation, extracts were generated in lysis buffer prior to incubation with the indicated antibodies. Proteins were separated on 4-12% Tris-glycine gradient gels (Invitrogen). To examine E2 charging, cells were lysed in 50 mM MES, 150 mM NaCl, 0.2% Nonidet-P40, protease inhibitors (Roche, Indianapolis, Ind.) (pH 3-5 as indicated) buffer and cleared by centrifugation. Ten μg of extract was subjected to non-reducing Novex 4-12% Bis-Tris gel prior to immunoblotting. In cases where RNAi was employed, cells were transfected with the indicated siRNAs (33 nM) using Oligofectamine (Invitrogen). After 72 hours, cells were lysed as described above to examine E2 charging. The sequences of siRNAs are provided herein. In order to examine thioesters between ubiquitin and Flag-HA-Uba6, cells expressing Flag-HA-Uba6 (wild-type or the C625A mutant) were lysed in 50 mM MES, pH 4.5, 150 mM NaCl, 0.2% Nonidet-P40, protease inhibitors and extracts subjected to immunoprecipitation with anti-HA-agarose. Immune complexes were washed with lysis buffer (pH 4.5), eluted with HA peptide, and then subjected to electrophoresis on a 412% Bis-Tris gel in the absence of reducing agent. In some cases, samples were treated with 200 mM DTT prior to electrophoresis in order to reduce thioester bonds. Gels were transferred to PVDF and probed with the indicated antibodies or, in some cases, bands were excised and subjected to mass spectrometry as described below.

Tissue Culture

Tissue culture cells were grown in Dulbecco's Modified Eagle Medium at 37° C. in 5% CO₂. To generate cell lines stably expressing Flag-HA-Uba6 or relevant mutants, the Uba6 ORFs were recombined into either pHAGE-CMV-Flag-HA-GAW or pHAGE-PGK-Flag-HA-GAW and viruses were packaged using standard lentivirus packaging procedures. Viral supernatants were used to infect 293T cells at a multiplicity of infection of approximately 0.5. Cells were selected for integration using puromycin.

Expression of Proteins in E. coli

For expression of GST-ubiquitin and ubiquitin-like proteins in E. coli, BL21/DE3 cells (Novagen, Madison, Wis.) were transformed with the appropriate expression plasmid (pDEST-27, Invitrogen) and plasmids selected using carbocyclin. Cells were grown to 0.8 OD and induced with 0.4 mM IPTG. After three hours, cells were harvested, lysed in 50 mM Tris-HCl (pH 7.5), 150 mM NaCl, 1 mM EDTA, 0.5% NP40, and subjected to centrifugation at 12,000 g. Extracts were subjected to purification using GSH-Sepharose (Pharmacia, Piscataway, N.J.). After washing the beads with lysis buffer, proteins were eluted using 0.1 M Tris-HCl, pH 8, and 40 mM glutathione. To translate E2s in vitro, coding sequences in vectors containing a T7 promoter were transcribed and translated using an E. coli in vitro translation system (Promega) in the presence of ³⁵S-methionine. Proteins were then employed in thiolester assays as described below.

Expression of Proteins in Insect Cells

For expression in insect cells, ORF clones were recombined with baculoviral sequences using Bac-N-Blue (Invitrogen) via co-transfection of Sf9 cells and viral supernatants isolated three days after transfection. Viral stocks were amplified in Sf9 cells. For protein production, Sf9 cells were infected with viral stocks and cells were lysed after 40 hours using 50 mM Tris-HCl (pH 7.5), 150 mM NaCl, 1 mM EDTA, and 0.5% NP40 in the presence of proteasome inhibitors (Roche, Basel, Switzerland). Extracts were cleared by centrifugation and lysates were incubated with either anti-Flag (Sigma, St. Louis, Mo.) or GSH-Sepharose (Pharmacia, Piscataway, N.J.) resins. Resins were washed with lysis buffer prior to elution with either Flag peptide (Sigma) or glutathione as described above. Proteins were dialyzed against 50 mM Tris-HCl, 50 mM NaCl, and 50% glycerol at 4° C. for 2 hours and stored at −80° C.

Ubiquitin Activation and Transfer Reactions

To examine ubiquitin activation, Ube1 or Uba6 (100 ng) was incubated in 50 mM Tris-HCl, pH 7.5, 50 mM NaCl, and 4 mM ATP for the time indicated above in the presence of 10 μg ubiquitin or ubiquitin-like protein (final volume of 101). Reaction mixtures were subjected to non-reducing SDS-PAGE at the indicated times. Gels were either stained with Coomassie or subjected to immunoblotting with the antibodies indicated above.

To examine transfer of ubiquitin from Uba6 to an E2, a thiol ester assay was performed. Reactions were performed as described above except that 2 μl of in vitro translated and ³⁵S-methionine labeled E2 was added. After the indicated time, reaction mixtures were subjected to non-reducing SDS-PAGE and dried gels subjected to autoradiography to visualize the E2 protein and their thiol ubiquitin forms. In some experiments, ubiquitin lacking lysine residues (UbKO) was employed.

Expression of Proteins in Mammalian Cells

In order to examine ubiquitin conjugates for Uba6 in mammalian cells, the indicated cell lines stably or transiently expressing Uba6 or mutants were lysed. Cleared lysates were subjected to immunoprecipitation using anti-HA resin. After washing the resin with lysis buffer, proteins were subjected to non-reducing SDS-PAGE and immunoblotting with the indicated antibodies. In certain cases, gels were stained with Coomassie and proteins excised prior to mass spectrometry at the Taplin Mass Spectrometry Core Facility (Harvard Medical School).

General Methods

Ubiquitin activation assays contained 8 nM E1, 100 nM GST-UBL and 2 mM ATP in 50 mM Tris-HCl, 5 mM KCl, 5 mM MgCl₂ for 15 min at 30° C. (total volume 10 μl). Reactions were quenched with 2× Laemli sample buffer lacking reducing agent, and subjected to non-reducing 4-12% Tris-glycine gel and immunoblotting. To examine E2 charging, the indicated E2 was in vitro translated using a bacterial S30 extract (Promega, Madison, Wis.) in the presence of ³⁵S-methionine. E2 (1 μl) was incubated with 40 nM EL, 25 μM KO ubiquitin (Boston Biochem, Waltham, Mass.), 2 mM ATP (15 min, 30° C.) (10 μl total volume) before 4-12% Tris-glycine gel/autoradiography.

For immunoprecipitation, cell extracts were generated in pH 7.5 lysis buffer [50 mM Tris-HCl, pH 7.5, 150 mM NaCl, 0.5% Nonidet-P40, protease inhibitors (Roche, Indianapolis, Ind.)] or pH 4.5 lysis buffer (50 mM MES, pH 4.5, 150 mM NaCl, 0.2% Nonidet-P40, protease inhibitors). Proteins were separated on non-reducing 4-12% Tris-glycine or 4-12% Bis-Tris gradient gels prior to blotting or mass spectrometry. To examine E2 charging in vivo, cells were lysed in 50 mM MES (pH 3 to 5 as indicated), 150 mM NaCl, 0.2% Nonidet-P40, protease inhibitors and cleared by centrifugation. Extracts (10 μg) were subjected to non-reducing 4-12% Bis-Tris gel prior to immunoblotting. Where indicated, extracts were boiled (5 minutes) with 200 mM dithiothreitol (DTT). For RNAi, cells were transfected using Oligofectamine (Invitrogen, Carlsbad, Calif.). After 72 hours, cells were lysed as described herein. siRNA sequences: siUba6-1, CCTTGGAAGAGAAGCCTGATGTAAA; siUba6-2, ACACTGAAGTTATTG TACCGCATTT; siUba6-3, GGGATCGATGGACCGTACATGGAAA; siUbe1-1, GAGAAGCTGGGCAAGCAGAAGTATT; siUbe1-2, CCGACAGCTTGACTCC TACAAGAAT; siUbe1-3, TCCTCAACTTGGCCCTGCCTTTCTT.

Example XI References

-   Gu, X. et al., “Cloning and characterization of a gene encoding the     human putative ubiquitin conjugating enzyme E2Z (UBE2Z),” Mol. Biol.     Rep., 34:183-188 (2006). -   Pickart, C. M. & Eddins, M. J., “Ubiquitin: structures, functions,     mechanisms,” Biochim. Biophys. Acta, 1695:55-72 (2004). -   Huang, D. T., Walden, H., Duda, D. & Schulman, B. A.,     “Ubiquitin-like protein activation,” Oncogene 23:1958-1971 (2004). -   Kerscher, O., Felberbaum, R. & Hochstrasser, M., “Modification of     proteins by ubiquitin and ubiquitin-like proteins,” Annu. Rev. Cell     Dev. Biol., 22:159-180 (2006). -   Ciechanover, A., Elias, S., Heller, H. & Hershko, A. “‘Covalent     affinity’ purification of ubiquitin-activating enzyme,” J. Biol.     Chem., 257:2537-2542 (1982). -   Haas, A. L., Warms, J. V., Hershko, A. & Rose, I. A.,     “Ubiquitin-activating enzyme. Mechanism and role in     protein-ubiquitin conjugation,” J. Biol. Chem., 257:2543-2548     (1982). -   Hershko, A., Heller, H., Elias, S. & Ciechanover, A., “Components of     ubiquitin-protein ligase system. Resolution, affinity purification,     and role in protein breakdown,” J. Biol. Chem., 258:8206-8214     (1983). -   Pickart, C. M. & Rose, I. A., “Functional heterogeneity of ubiquitin     carrier proteins,” J. Biol. Chem., 260:1573-1581 (1985). -   Pickart, C. M., “Back to the future with ubiquitin,” Cell,     116:181-190 (2004). -   Lake, M. W., Wuebbens, M. M., Rajagopalan, K. V. & Schindelin, H.,     “Mechanism of ubiquitin activation revealed by the structure of a     bacterial MoeB-MoaD complex,” Nature, 414:325-329 (2001). -   Duda, D. M., Walden, H., Sfondouris, J. & Schulman, B. A.,     “Structural analysis of Escherichia coli ThiF,” J. Mol. Biol.,     349:774-786 (2005). -   Lehmann, C., Begley, T. P. & Ealick, S. E., “Structure of the     Escherichia coli ThiS-ThiF complex, a key component of the sulfur     transfer system in thiamin biosynthesis,” Biochemistry, 45:11-19     (2006). -   Walden, H., Podgorski, M. S. & Schulman, B. A., “Insights into the     ubiquitin transfer cascade from the structure of the activating     enzyme for NEDD8,” Nature, 422:330-334 (2003). -   Lois, L. M. & Lima, C. D., “Structures of the SUMO E1 provide     mechanistic insights into SUMO activation and E2 recruitment to E1,”     Embo. J., 24:439-451 (2005). -   Bencsath, K. P., Podgorski, M. S., Pagala, V. R., Slaughter, C. A. &     Schulman, B. A. Identification of a multifunctional binding site on     Ubc9p required for Smt3p conjugation,” J. Biol. Chem.,     277:47938-47945 (2002). -   Huang, D. T. et al., “Structural basis for recruitment of Ubc12 by     an E2 binding domain in NEDD8's E1,” Mol. Cell., 17:341-350 (2005). -   Huang, D. T. et al., “Basis for a ubiquitin-like protein thioester     switch toggling E1E2 affinity,” Nature, 445:394-398 (2007). -   Finley, D., Ciechanover, A. & Varshavsky, A., “Thermolability of     ubiquitin activating enzyme from the mammalian cell cycle mutant     ts85,” Cell, 37:43-55 (1984). -   Ciechanover, A., Finley, D. & Varshavsky, A., “Ubiquitin dependence     of selective protein degradation demonstrated in the mammalian cell     cycle mutant ts85,” Cell, 37:57-66 (1984). -   McGrath, J. P., Jentsch, S. & Varshavsky, A., “UBA 1: an essential     yeast gene encoding ubiquitin-activating enzyme,” Embo. J.,     10:227-236 (1991). -   Odorisio, T., Mahadevaiah, S. K., McCarrey, J. R. & Burgoyne, P. S.,     “Transcriptional analysis of the candidate spermatogenesis gene     Ube1y and of the closely related Ube1x shows that they are     coexpressed in spermatogonia and spermatids but are repressed in     pachytene spermatocytes,” Dev. Biol., 180:336-343 (1996). -   Haas, A. L. & Bright, P. M., “The resolution and characterization of     putative ubiquitin carrier protein isozymes from rabbit     reticulocytes,” J. Biol. Chem., 263:13258-13267 (1988). -   Komatsu, M. et al., “A novel protein-conjugating system for Ufm1, a     ubiquitin-fold modifier,” Embo. J., 23:1977-1986 (2004). -   Walden, H. et al., “The structure of the APPBP1-UBA3-NEDD8-ATP     complex reveals the basis for selective ubiquitin-like protein     activation by an E1,” Mol. Cell, 12:1427-1437 (2003). -   Eletr, Z. M., Huang, D. T., Duda, D. M., Schulman, B. A. & Kuhlman,     B., “E2 conjugating enzymes must disengage from their E1 enzymes     before E3-dependent ubiquitin and ubiquitin-like transfer,” Nat.     Struct. Mol. Biol., 12:933-934 (2005). -   Booth, J. W., Kim, M. K., Jankowski, A., Schreiber, A. D. &     Grinstein, S., “Contrasting requirements for ubiquitylation during     Fc receptor-mediated endocytosis and phagocytosis,” Embo. J.,     21:251-258 (2002). -   Shringarpure, R., Grune, T., Mehlhase, J. & Davies, K. J.,     “Ubiquitin conjugation is not required for the degradation of     oxidized proteins by proteasome,” J. Biol. Chem., 278, 311-8 (2003). -   Chen, X. et al., “N-acetylation and ubiquitin-independent     proteasomal degradation of p21(Cip1),” Mol. Cell, 16:839-847 (2004). 

1. A method for inhibiting a Uba6 activity comprising: contacting Uba6 with a compound that inhibits formation of a ubiquitin-adenylate intermediate.
 2. The method of claim 1, wherein the compound binds an adenylation domain of Uba6.
 3. The method of claim 1, performed in vitro or in vivo.
 4. The method of claim 1, performed in tissue culture cells.
 5. A method for inhibiting a Uba6 activity comprising: contacting Uba6 with a compound that inhibits thiol esterification of Uba6.
 6. The method of claim 5, performed in vitro or in vivo.
 7. The method of claim 5, performed in tissue culture cells.
 8. A method for inhibiting a Uba6 activity comprising: contacting Uba6 with a compound that inhibits transfer of ubiquitin to a ubiquitin conjugating enzyme.
 9. The method of claim 8, wherein the compound binds a Ub1 domain of Uba6.
 10. The method of claim 8, performed in vitro or in vivo.
 11. The method of claim 8, performed in tissue culture cells.
 12. The method of claim 8, wherein the ubiquitin conjugating enzyme is selected from the group consisting of: E2C, E2D1, E2D2, E2D3, E2D4, E2E1, E2G, E2S, E2T and E2Z.
 13. The method of claim 8, wherein the ubiquitin conjugating enzyme is E2Z.
 14. A method for inhibiting ubiquitin activation comprising: contacting Uba6 with a compound that inhibits a catalytic cysteine domain of Uba6.
 15. The method of claim 14, performed in vitro or in vivo.
 16. The method of claim 14, performed in tissue culture cells.
 17. A method of reducing a Uba6 activity in an organism in need thereof comprising: administering to the organism one or more siRNAs complementary to a portion of a Uba6 mRNA.
 18. The method of claim 17, wherein the siRNA is an RNA sequence selected from the group consisting of SEQ ID NO:1, SEQ ID NO:2 and SEQ ID NO:3.
 19. The method of claim 17, wherein the portion of the Uba6 mRNA encodes a ThiF domain, a catalytic cysteine domain, an adenylate domain or a ubiquitin-like domain.
 20. The method of claim 17, wherein the organism is a human.
 21. A method of reducing ubiquitination in an organism in need thereof comprising: administering to the organism one or more compounds that inhibits one or more Uba6 activities in the organism.
 22. The method of claim 21, wherein the compound is an antibody or an siRNA.
 23. The method of claim 21, wherein the organism is a human.
 24. A method of identifying a compound that inhibits charging of E2Z comprising: providing a sample including E2Z and ubiquitin; contacting the sample with the compound; contacting the sample with Uba6 or a biologically active portion thereof, and determining whether the ubiquitin is bound to the E2Z in the presence of the compound, wherein the ubiquitin is not bound to the E2Z if the compound inhibits charging of E2Z.
 25. The method of claim 24, further comprising visualizing E2Z on an SDS-PAGE gel.
 26. A method of identifying a compound that inhibits a Uba6 activity comprising: providing a sample including a ubiquitin conjugating enzyme and ubiquitin; contacting the sample with the compound; contacting the sample with Uba6 or a biologically active portion thereof, and determining whether the ubiquitin is bound to the ubiquitin conjugating enzyme in the presence of the compound, wherein the ubiquitin is not bound to the ubiquitin conjugating enzyme if the compound inhibits the Uba6 activity.
 27. The method of claim 26, wherein the ubiquitin conjugating enzyme is selected from the group consisting of: E2C, E2D1, E2D2, E2D3, E2D4, E2E1, E2G, E2S, E2T and E2Z.
 28. The method of claim 26, wherein the ubiquitin conjugating enzyme is E2Z.
 29. A method of identifying a compound that inhibits a Uba6 activity comprising: providing a sample including ubiquitin; contacting the sample with the compound; contacting the sample with Uba6 or a biologically active portion thereof, and determining whether the ubiquitin is bound to the Uba6 or the biologically active portion thereof in the presence of the compound, wherein the ubiquitin is not bound to the Uba6 if the compound inhibits the Uba6 activity.
 30. The method of claim 29, wherein the ubiquitin is bound to the Uba6 or the biologically active portion thereof via thiol conjugation.
 31. The method of claim 29, wherein the ubiquitin is immobilized.
 32. An RNA sequence having at least about 70% identity to a nucleic acid sequence selected from the group consisting of SEQ ID NO:1, SEQ ID NO:2 and SEQ ID NO:3, wherein the RNA sequence can inhibit a Uba6 activity.
 33. The RNA sequence of claim 32, wherein the Uba6 activity is selected from the group consisting of ubiquitin activation, ubiquitin-adenylate intermediate formation, ubiquitin thiol esterification, ubiquitin transfer to a ubiquitin-conjugating enzyme and ubiquitination of a target polypeptide.
 34. The RNA sequence of claim 32, wherein the RNA is siRNA.
 35. An RNA sequence selected from the group consisting of SEQ ID NO:1, SEQ ID NO:2 and SEQ ID NO:3.
 36. the RNA sequence of claim 35, wherein the RNA sequence can inhibit a Uba6 activity.
 37. The RNA sequence of claim 36, wherein the Uba6 activity is selected from the group consisting of ubiquitin activation, ubiquitin-adenylate intermediate formation, ubiquitin thiol esterification, ubiquitin transfer to a ubiquitin-conjugating enzyme and ubiquitination of a target polypeptide.
 38. The RNA sequence of claim 35, wherein the RNA is siRNA. 